Like a lot of people, I'm not thrilled with version 4.0 of the iPhone Twitter client; but what bugs me the most is a small thing, namely it's way of handling Instapaper. For one thing, you have to click on the article before saving it; that isn't particularly inefficient (you don't have to wait for the article to actually load), but it feels conceptually wrong to me, since I'm explicitly deciding not to read the article right then. But what's wrong on a more pragmatic level is that it no longer saves the text of the referring tweet in Instapaper, making it almost impossible for me to credit the referrer if I decide to link to that saved article on my linkblog.
I gave Tweetbot a try; it saved the text of the referring tweet but not its author, which is almost as useless for citation purposes. And Tweetbot has a whole other host of issues around state and strange ways of tapping (double-tapping, tap-and-hold). So I think I'm going back to the official client; but I miss Tweetie. It's certainly surprising how much of a difference these apparently small design decisions make to me.