Ballmer doesn't strike me as an idiot and definitely not bland. He's one of the more colorful tech personalities. MS's almost unassailable lead in enterprise could be attributed to him and the pivot to cloud could not have happened without this. But he definitely fumbled hard on mobile (Windows Phone), Surface (IIRC the initial ARM laptop was a major flop and had a close to 1B+ writeoff) and the disaster that was the Nokia acquisition. I'd say he left at the right time, just as it was becoming clear that MS's bets on Windows Phone and hardware in general weren't paying off.