I'm been a fan for more years than I care to remember, so wasn't sure at all about 'Frankenyouth' (as we unkindly dubbed him) taking over.
At the beginning he seemed to be trying to hard to channel David Tennant, and all that manic leaping about got a touch wearisome. Despite the woeful wartime episode, he seems to have settled in a little more now, though I'm still not sure exactly what Amy and dimwit boyfriend are for.
Perhaps he needs a series or so to bed in, I'll reserve judgement until I've seen the complete run of this one.
I actually liked John Simm as the Master, but then I thought he would have made a great Doctor too - oh well, maybe next time