The thing about real-world robots is that they suck. I was raised on a diet of Transformers and Terminator, where stomping machines clank around, whirr into supercool jetplanes and goop through helicopter windows. Back in reality, scientists waste their time fannying about with particles they can't even see and lack the basic skills to make a robot walk like a human. Screw that.

In the glorious gib-soaked fantasy land of Team Fortress 2, robots are as they should be. The latest free update, Mann Vs. Machine, is a giant robot death-pit filled with endless waves of death-dealing machinery, just lining up to be trashed.

Mann. Vs Machine brings much that's new to TF2, which at the ripe old age of five years still looks and plays better than any other multiplayer shooter out there. The first thing is co-op, or 'PVE' as the PC kids call it. Given we've been shooting other people as RED and BLU forever, it feels unusually civil. The kind of thing you might crack out a top hat for.

The giant Heavy is a right bastard, and in the middle of a group of gnat-like Scouts it's almost impossible to keep your focus on taking him down. I don't even want to talk about the robot Medics.

Your team, on three custom maps with two different configurations each, spawns next to an upgrade station. This has everything covered, all the classes and all the possible weapons, meaning there's more options than HAL could process. Take as an example the Pyro's flamethrower: you can boost the damage, the burn damage, the burn duration, the ammo, or the knockback on its airblast. The Engineer can unlock an extra mini-sentry, boost the firing speed of his main gun or increase its armour, and increase his own metal-toting capacity.

All of this is enabled by, and in the service of, killing loads of robots. Each map is basically a big corridor with alternate routes - you spawn at one end, and the robots flood in from the other. There are absolutely loads of them. Early waves are piles of irritating Scout robots with the odd Heavy or Soldier thrown in. Here it's mainly about clearing them efficiently and hoovering up the dropped cash - which is what gets you the upgrades, though it can also be spent on slightly quicker respawns.

"It's part of Valve being Valve that such a well thought-out and substantial co-op mode... is doomed to receive such empty praise as 'effortless'. This is a hell of a piece of work."

This is merely acclimatisation, because Mann Vs. Machine quickly stops pussyfooting around and gets the kitchen sink involved. Huge, slow-moving bomb trains with incredible amounts of health, paired off with giant Heavy robots being healed by a team of über-Medics, scores of Scouts, Pyros and Spys backed up by the laser sights of a Sniper troupe. I haven't completed all seven waves once.

It's part of Valve being Valve that such a well thought-out and substantial co-op mode, a design that mixes tower defence tactics and escalating enemy waves into the best online shooter out there, is doomed to receive such empty praise as 'effortless'. This is a hell of a piece of work, with the only black mark an often laborious matchmaking system, though it seems to be improving in efficiency. Early reports that a 'Mann. Up' ticket, a 59p in-game purchase, would result in quicker matchmaking were not borne out in my experience. Though frankly, even if they were, 59p is nothing to pay for this.

One of Mann Vs. Machine's cutest touches is that the robots taunt after killing you, and the only thing worse than a gloating human is a gloating AI.

But Mann Vs. Machine is, like Team Fortress 2 itself, free to play. It's now just over a year since the switch to this business model, which looks like a winner from every angle. This superb game now earns more than 12 times as much for its makers than it had done before (and that's an old estimate), while the huge player-base enjoys the random delights of its princely new meta-game: item-collecting and hat-hoarding.

Of all the changes in Team Fortress 2 since launch, perhaps the role of items is the most profound. From today's perspective the launch classes now seem so vanilla because a steady series of free updates has added so much specialisation. At the extremes, characters can have new roles entirely - like playing the Demoman as a crazy, charging melee class.

These items are randomly 'found' - basically, every couple of hours played will see you pick up an item, with almost anything a possibility. Sometimes these will be 'Supply Crates', which rather divide people. Crates require a £1.99 key from the store to unlock, though usually the loot within is pretty hot. So there's a constant drip of stuff into your inventory, and the crates are easy enough to trade if you don't want to pay for anything. I've spent about £20, mostly on keys, and don't regret a penny. It feels like tipping the best bartender in the world when all the drinks are on the house.

The customisation that comes from these items is now TF2's beating heart. It is a game where you have favourites. And when you spend a certain amount of time in a game, making that class absolutely fit is an incredibly important thing.

In-jokes abound on the levels, including an impressive display of the Hale family's hat collection.

Half of it is practical, and half cosmetic. The people who complain about micro-transactions and spending money on hats do not understand. I was playing TF2 with my mate Owen and he had a weapon I wanted. He traded me the sword or whatever, and I said 'right, shall we get into a match?' There was a brief silence over Skype, before:

"Come on then, what hats you got?"

"Hats?"

"Yeah, we're men aren't we? Let's have a look."

"I've spent about £20, mostly on keys, and don't regret a penny. It feels like tipping the best bartender in the world when all the drinks are on the house."

So we got our hats out. He had one I was keen on, a press hat, but wanted to keep it. A couple of days later, I get a wrapped-up gift when I log into TF2. You know what was in it.

Now, that wrapping paper cost him £1.69. Was it a waste of money? All it did was let him send a hat to me immediately as a surprise, as opposed to waiting a few days then sending it as is. There's not really anything of tangible value there. But that cash was paid over in the same spirit you'd buy a chintzy keyring on holiday, because you think someone back home will find it funny. It's not that the thing itself is of value, so much as the fact you like the idea of someone else seeing it.

Mann Vs Machine would be great for anyone looking to learn the Engineer class, though you'd have to bear in mind how ludicrous his sentry guns are here once upgraded.

Not everyone plays Team Fortress 2 with their real-life mates, of course, but that's the best way to do it. It's one of gaming's pinnacles, that little blurring between a shared joy like TF2 and your life. Some people will always think it's stupid to spend hard cash on virtual wrapping paper for a virtual item. I respect that. But just because something doesn't physically exist doesn't mean it lacks value.

The greatness of Team Fortress 2's free-to-play model lies in the fact that the game feels permanent - like you'll be playing it for a long time, your mates will be playing it for a long time, and your stash will be around for a long time. This is almost entirely down to the way Valve has husbanded it from launch - constant minor tinkering, total openness to community content and a steady stream of substantial updates. At launch, Team Fortress 2 was an amazing game. Today it's multiple times better, an incredible achievement and vivid monument to the hard work and imagination of its developers.

Team Fortress 2 has never stopped giving, never stopped growing, and it's hard to see when it will. Which is why an update as amazing as Mann Vs Machine feels a lot like business as usual.