I have been using IE7 beta 2 public preview for a while and found it OK. The good thing about it is low memory usage and pretty quick. It doesn't support meebo by the way, if you login to your IM via meebo it will tell you that passwords are not encrypted because you are using IE7. This is OK for Opera 9 (preview 2), Firefox and IE6.
Opera 9 preview 2 has a good start, it supports fully gmail now. See screenshot below. In version 8 the wysiwyg editor wasn't available. I still can see little flaw such as display of attached photos, I'm sure Opera will have that sorted out later. Too bad Opera 9 doesn't support meebo.
Both Opera 9 and IE7 don't support Yahoo Mail beta. Opera also doesn't support Windows Live Mail beta but will display it. Opera 9 will prompt you to go back to old version of Yahoo mail.
I guess nobody is perfect, yet!