I've never been a big fan of tabbed browsing but after using it for a couple weeks I'm getting used to it. I haven't had any performance problems with they system in general but it has locked up and crashed (not the whole system though) at least twice since 11/7...IE6 didn't crash twice in the previous 6 months.
I don't care for the button placement either. The refresh and stop buttons are just oddly placed. They should be next to the back/forward buttons IMO.
I haven't spent a lot of time with it yet to see if any of that stuff is customizable but I'm thinking probably not. Microsoft doesn't seem to build in that level of user friendly customization.
But, if you guys don't like the interface in IE7, wait till you see Vista!!