IE7 is still new and while there are many sites it will just work without any issues there are many that it will just not work and you will get a “I don’t know what the heck that browser is” error. (There are a few who handle this quite gracefully - e.g. I am on the Yahoo Mail Beta and they say they don’t support it, but if you want to give it a shot - go ahead).
If some of these sites who do not want to party with IE 7 are on your must have and you cannot live without them, then I would recommend to check out the User Agent String utility from Microsoft . This opens a new IE 7 instance that reports to being as IE 6.0. And it runs on most of the flavors of Windows (XP, 2003 and Vista).