Currently running IE7 (hadn't installed IE8 after noticing that it seemed slower than IE7 on some of the systems).
FireFox -- pretty good. Not many problems with it. Have noticed that when viewing overseas websites it does a better job of detecting language settings that IE (so I'm not reaching the language setting as often as I do with IE). NoScript plug-in works great.
Chrome -- not bad, though I did run into some compatibility issues (on a few websites it went spectacularly wrong). Does seem more responsive than IE7 or IE8. The minor gripe I've got with it is that on older systems (basically computers with single-core CPU) the system load shoots up noticibly.
Opera -- has the smallest "footprint" so it works great on older and slower systems with less memory. There are some compatibility issues with a few websites. Have not noticed any problems with UT.