Ballmer was a math genius, he was also a spreadsheet whiz and knew as much as a CFO did at anytime. His memory was that of a thousand elephants, and could recite forecasts, actuals and numbers for multiple years in one go.
Microsoft played in the enterprise space, and Ballmer was a marketing genius when it came to enterprise positioning. I credit him with driving the attach revenue concept within the enterprise. Companies that bought Windows, bought office, bought Exchange server, bough maintenance and more.
Even more, he was a relationship marketing genius. He had a photographic memory and remembered names of people he would meet once and recall entire conversations after months/years. And this was globally, he took the company global in a very aggressive way.
He rewarded people, both Bill and Steve weren't stingy about doling out stock - unlike Jobs. This kept a strong talent pool of A players at Microsoft.
He had a strong penchant for the enterprise and where he started faltering was when the Internet started maturing and consumer experiences started converging with the enterprise.
Nevertheless, this man took Microsoft from $15B to $70B in revenue and you can't belittle that.