I think he actually bought well for us (RSC being the exception); Bellamy, Zabba, Kompany, Barry, Lescott, De Jong - can't argue with any of those. Even Robinho was good for a period of time.
His problem is that he's just a crap manager and coach, and that is exacberated by his apparent unwillingess to accept responsibility - it's always the fault of 'decisions'. Every time I see him in a post-match interview, I just wait for the inevitable lamenting of decisions.
Top managers can be arrogant and single-minded (and arguably need to be), but they also need to be able to recognise their mistakes and shortcomings so that they can improve on them. Those that can't, fail.
He failed at City because we were crap under him, despite having some excellent players. Mancini showed him up. Simple as that.