I can only speak from my experiences. I never played CoD3, and I've only played CoD on 360. W@W was fantastic, and Black Ops generally plays quite smoothly for me. In fact, Black Ops is easily my favorite in the series so far. As such, I find Treyarch to be a perfectly competent development studio. Yeah, I get that they kinda shit the bed with the PC version, at least at launch, but let's not forgot which studio decided to shit directly on their PC fans with MW2 (the whole dedicated server fiasco).
(And holding CoD3 against Treyarch is simply ignorant; the fact that the game even exists is something of a miracle given the ridiculously short dev time the studio had to make that game. Basically, blame Kotick, not Vahn.)
To be honest, I still feel like CoD4 was "lightning in a bottle." Both studios have been trying desperately to repeat that level of genius ever since 2007, and neither of them have gotten back to that point. W@W was, essentially, a CoD4 mod. MW2 was an entertaining game, yet horribly unbalanced to the point of being broken (and it was a ridiculous glitch-fest too, in case you forgot). I think Black Ops has come the closest; it's taken the best ideas from MW2, W@W, and CoD4, and it's introduced a few new ideas of its own.
tl;dr: The way I see it:
Infinity Ward is still riding on the wave of good feelings from CoD4, but is trending downwards.
Treyarch is still riding on the wave of shit from CoD3, but is trending upwards.