A decade ago, phone makers were in a race to see who could build the tiniest mobile device — a flip phone that could fit in the palm of your hand. Now the pendulum has swung the other direction, and the competition is to see who can build the biggest handheld computer that can still technically qualify as a phone.

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In the ever-expanding phablet category, the Nokia Lumia 1520 is currently the biggest of them all, with its 6-inch display winning the spec battle over the 5.7-inch Samsung Galaxy Note 4 and Apple's new 5.5-inch iPhone 6 Plus. (Sony's Experia Z Ultra is larger, at 6.4 inches, but you can't buy it in the U.S. from any carrier.)

I've had a chance to tinker with the 1520 over the past few months, but recently purchased one of my own and got my hands on a review unit supplied by Microsoft. Having two of these gargantuan devices in the house meant that my wife and I could both set aside our regular phones and use these beasts for a while.

What follows are personal impressions from roughly a week of hands-on experience, along with some thoughts about the unique nature of the Windows Phone experience.

Although it's tempting to focus on the distinctive 1520 hardware (did I mention it's really big?), it's impossible to talk about a modern mobile device without considering all its component parts: the hardware and the operating system and apps. And, of course, the carrier, because for better or worse the relationship between the Lumia 1520 and its exclusive carrier is a story in itself.

The hardware

The 1520's 6-inch Full HD display makes a dazzling first impression. And that satisfying impression persists over time.

I am not entirely sure what ClearBlack is, but the technology in this IPS display sure makes it easy to read in a broad range of lighting conditions. That's true even in sunlight, where other non-Lumia smartphone screens, including the otherwise exemplary HTC One M8 (Android and Windows versions) are basically invisible. (The Lumia Icon is also an extraordinary performer in bright environments, I should note.)

At 368 pixels per inch, text and pictures alike are extremely sharp. You really notice the difference in clarity when you switch back to a device with a more conventional display. If you normally need reading glasses for a smartphone display, you might be surprised to find this one big and sharp enough to read without assistance. Neither of us experienced fatigue even during long sessions of reading.

After using the 1520 for four days, my wife declared it the perfect traveling device, sitting in the "sweet spot" between phone and tablet. And even though it's too big to fit comfortably in most pockets, it tucks quite nicely into a purse or shoulder bag.

Judy described the 1520 as feeling like "the lightest tablet I've ever used." The screen was more than adequate with the Windows Phone Kindle app (as long as you don't mind turning pages more frequently). It's also big enough to use comfortably for maps, for looking up restaurant reviews, and for general web browsing. It weighs about 30 grams more than its Samsung or Apple rivals but is still impressively thin.

The weird thing is that you could almost pretend this thing doesn't have a phone app at all. Leave the SIM out completely and this would still be a great movie and music player.

It certainly can hold a lot of media. The availability of the MicroSD card slot means storage is expandable at a reasonable cost. That's a noteworthy advantage over the iPhone 6 Plus, whose storage is set when you buy it and can never be expanded.

That expansion capability gives the 1520 a big edge in price: the 16GB model starts at $585, which is $165 less than the cost of the equivalent iPhone 6 Plus. If you add a 64 GB or 128 GB MicroSD card to the 1520, the price gap gets even more noticeable, with the 1520 plus 128 GB MicroSD card costing $250 less than the top-of-the-line iPhone 6 Plus with 128 GB.

I didn't spend a lot of time with the camera on this phone. I've been spoiled by the delightful images the Lumia Icon captures, and the 1520 is identical in specs to that device. The photos I have taken with the 1520 have been crisp and sharp. The Nokia engineers who built the cameras in all of the Lumia flagship devices know what they're doing.

Inside the oversize case is a 3400 mAh battery, which helped it achieve all-day battery life for me. Unfortunately, AT&T doesn't include wireless charging support in this or any other Lumia devices that it sells. That's a feature of the Lumia Icon that I really miss.

The network

One of the greatest frustrations of the Lumia 1520 is its position as an exclusive on a single carrier, AT&T.

That's a problem with the whole Lumia lineup, in fact. This hardware design might be perfect for you, except it doesn't work with your carrier's network.

Unlike the iPhone 6 Plus, the Lumia 1520 isn't available on your choice of carriers. It's locked to AT&T in the U.S., and there's no international version. [Update: Actually, at least two international versions are available, but their capabilities vary, especially with regard to support for LTE frequencies from U.S. carriers. Look for the model numbers RM-937 and RM-938, aka 1520.3.]

You say you're an AT&T customer and the phone is paid in full and you've been a customer for months, maybe years? you might be able to convince AT&T to unlock it for you, but don't count on that.

If, like me, you live in an area that's poorly served by AT&T, that means the 1520 starts with two strikes against it. And Verizon, which offers rock-solid service in my hometown, has nothing in the Windows Phone category that is even close to this size.

Meanwhile, on Verizon I have the option of the Lumia Icon, which I've been using for about six months. It's a lovely piece of hardware, in the same size and weight class as the iPhone 6. In short, it's a bigger phone, not a phablet. And sorry, AT&T customers, it's not available for you.

Windows on a Phone?

The Windows Phone experience is nothing like that of its rivals. The core navigation principle of both iOS and Android is icons of uniform size on a grid of home screens, with Android adding widgets for extra flair. Windows Phone uses live tiles, which combine program launching and notifications and pinned items into tiles of varying sizes on a single, scrolling home screen

The latest updates to the Lumia 1520 include the Lumia Cyan firmware and Windows Phone 8.1. I installed the free Preview for Developers app to get the latest OS updates, including the ability to organize tiles into folders.

Unlike in rival mobile OSes, opening a folder doesn't simply display a new screen full of icons identical in size and shape to those on the home screen.

Instead, tapping a folder slides it down, exposing its contents without forcing you to navigate away from the home screen. Inside the folder, you can have live tiles of varying sizes, just as you can on the home screen. Here, for example, is an expanded view of the folder I created to house various tools for managing the phone and my account.

Most of what I do is built around Microsoft services, including Office 365, Exchange Online for email, OneDrive for file storage, and the Xbox Music subscription service. If I used Gmail and Dropbox, I'd be frustrated at the absence of those two apps. In fact, the absence of almost anything Google is the most striking gap on a Windows Phone. Yes, you can connect a Gmail account via IMAP, but you can't do Google Hangouts or use Google Voice.

And if I had been conditioned by years of iPhone use to search for an app to solve every problem, I'd be stymied by the profound lack of apps in the Windows Phone Store.

I certainly don't feel deprived by that "app gap." Windows phones in general, and the Lumia 1520 in particular, are capable of handling just about any task I throw at them.

My email and calendar are synchronized smoothly thanks to Office 365.

The OneNote app for Windows Phone is polished and highly usable. I probably use it more than any other app.

The Nokia-built navigation tools under the Here brand—Here Maps, Here Drive+, and Here Transit—are exemplary. I'm especially happy with their ability to work offline, a feature we tested successfully in Europe last month. (The one time we got lost was, ahem, due to user error.)

The Nokia Camera app is rich without feeling complicated, and I am looking forwards to improvements coming to this device with the Lumia Denim update this fall.

All the big social media bases are covered, with apps for Facebook, Twitter (I use the superb third-party app Tweetium), Instagram, and WhatsApp.

And of course there's Cortana, which is still in beta but has already proved its value repeatedly.

The Lumia hardware includes a couple of cutting-edge features that are just beginning to appear in competing devices. Its motion sensor, coupled with the built-in Health & Fitness app, allows it to track your steps, Fitbit-style. And every flagship Lumia device contains NFC support, which is surprisingly useful for sharing photos between devices with a tap.

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In theory, that NFC chip also allows this device and other Lumia family members to function as digital wallets, paying for purchases with a tap.

The Lumia 1520 fits in a pocket, and thanks to its durable plastic shell it's unlikely to bend, a problem that is apparently plaguing its newest rival .

For day-to-day use, though, I'm likely to use the 1520 mostly as a tablet that doubles as a phone, with the device stashed in a portfolio-style case designed to fit in either the pocket of a sport coat or in a small bag. Several case designs I've seen include storage for a driver's license, cash, and credit cards, effectively replacing my wallet.

That's a trend I won't be surprised to see in other devices as well, assuming the phablet category lasts. After all, if the wristwatch can make a comeback, why can't the billfold?