Currently using: Firefox.
Upside: tabs, plugins, open source, standards compliant. No MS security issues. Not micro$oft. Nicer bookmark handling.
Downside:
slow to become active when task-switching back to it (1-2 secs, very irritating).
slow to return focus back to foreground when launching background tab
Roboform adapter doesn't work well
Can't use windowsupdate.microsoft.com with it.
Currently using: MyIE2.
Upside: great tab handling, very active development.
Downside:
irritating IE-derived behaviour
MS security issues.
not standards compliant.
Gave up on: CrazyBrowser (IE skin) was no longer under development.
Gave up on Opera after it was no longer smallest/fastest. And would crash on large form submissions.