I thought he was called "Keith O'Sullivan" until about two years ago when I saw his name in credits for the first time (seriously). My family totally ripped it out of me.
Just finished watching series 3. Pretty damn good still (e22 was a corker, as were a few of the early eps), but not in the same league as 1 and 2, IMHO. They're falling into the trap of letting the script slip and patching up the suspense with big-budget action sequences.
I don't think the format has been exhausted at all though - just needs better writing for s4 (and no more Palmer subplots perhaps). I hear it's being shot in London.