Evaluating the legacy of any CEO, especially one as storied and controversial as Microsoft's Steve Ballmer, is a challenge.

That's why I'm glad I had a chance to take it on. In late November 2013, I went to Redmond and interviewed Ballmer and a number of other current and former Microsoft leaders about Ballmer's last 13 years as CEO.

In August this year, I had the chance to do a 15-minute phone interview with Ballmer to talk about his early retirement announcement. That was the first time I'd been allowed to talk to him in the past 20 years. For the Ballmer story I wrote for Fortune -- "For Steve Ballmer, a lasting touch on Microsoft," -- I had 45 minutes with Ballmer in his office in Building 34, in what's expected to be his final press interview as CEO.

(And before anyone asks, I don't know who the next Microsoft CEO will be or when she or he will be named.)

Going into my "legacy" interview with Ballmer and, I wasn't expecting to learn a whole lot new. I've read lots about Ballmer and covered tens of his keynotes over the past couple decades. I knew he was a math whiz, scoring 800 on his math SATs. I knew he drove Fords because his dad used to work for Ford (and not because rumored CEO takeover candidate and Ballmer advisor Allan Mulally heads up that company). I knew he had his own private dining room in a steak house in Bellevue (and is partial to wedge salads).

Yet I was surprised by a few things I learned during my latest round of interviews, including:

The key role Ballmer played in helping Microsoft put its antitrust issues behind it





The effect of his decision to green-light and then champion the Xbox in Microsoft's goal to dominate the living room





His insistence on pushing the once-consumer-focused Microsoft to bet on the enterprise





The extent to which he took responsibility for the Longhorn debacle and the lessons he learned from it





And yes, that is bullet-proof glass behind us in the picture embedded above. Ballmer said Gates had it installed when he previously occupied the same office.

There have been highs, lows and a naggingly flat stock price since Ballmer took over the CEO job from the company's founder and only other CEO, Bill Gates. It was on Ballmer's watch that Microsoft cut 5,000-plus employees back in 2009. He admittedly made some bad investments and only narrowly escaped a potentially disastrous $45 billion one .

During Ballmer's tenure, Microsoft has changed in countless ways. When he took the helm in January 2000, Microsoft was a software company with about 39,000 people. Once Microsoft ends up digesting Nokia's devices and services business, by early next year, Microsoft will be a roughly 130,000-employee hardware, software and services business.

Ballmer, the CEO, has changed a lot, too. He's still the guy who will mime stomping some poor chump's iPhone during a company meeting for effect. But he's also one who is pragmatic enough to green-light Microsoft supporting the Linux operating system on Microsoft's Windows Azure cloud instead of bellowing about Linux being a "cancer" (as he did back in 2001).

"Most companies in an industry do a trick and they're finished, and then they sort of fade out," Ballmer told me. "We've built what I would call at least three tricks. Windows and Office was kind of a trick and our enterprise business is kind of a trick, and Xbox. A trick is sort of a success. Most companies in our industry sort of do one. We've done three. They've gone through wild rides. And we haven't just faded away. We've gone through the wild ride and come through it."

You can read here the article on Ballmer's legacy I wrote for Fortune. And watch my ZDNet "All About Microsoft" site this week and beyond for lots more posts on Ballmer's legacy.