I’ve been pretty enthusiastic about IE7 since it came out. It seems faster, has tabs, and handles most websites pretty well.
Recently, I’ve come to have a more negative view. Some of it is related to IE7’s UI quirks, some of it is just performance. Specifically:

• There’s not enough room for tabs (the bookmark buttons as well as the other buttons on there right just take room away)
• It never remembers the layout of the toolbar add-ons correctly. This is particularly irritating with 3+ toolbars.
• It’s slower than Firefox on the same machine
• It temporarily freezes a lot (probably a few times a day, for a good 30 seconds or so. reason unknown, and this is on a machine with tons of memory that’s not doing much else)
• Occasionally crashes. More than Firefox.

I’m also not convinced that memory usage with IE7 is much better. That was one of the original reasons I switched to it from FF 1.5, but 2.0 seems ok, and on a machine with 2GB, it just doesn’t matter that much.
Anyhow, the last straw was when it froze while I was taking my online traffic school test. Right in the middle of it. No fancy Javascript. No nothing. I had to kill it and restart my test.
The only real downside of going back to FF is that now I get the crappy version of the Outlook OWA interface again.