I'm fine with and actually like the constant guest appearances for the same reason I like the product placement: they're so blatant about it and go out of the way to make it seem forced that it becomes funny in and of itself. And when I praised the writing, it was mostly about the writing for Tracy. He's gotten a lot funnier over the last season I've watched; that's not to say he wasn't funny before, but they've really got a good handle on his character at this point.
The show is very good with what I think is one of the best ways to structure a TV joke: setting you up for what seems like a predictable line, and then giving you something different. Predictability is usually, in my opinion, a good indicator of the quality of a show; the less predictable it is, the better, at least to a certain point.