I'm a week and a half in, and I still dig gmail.. a ton.  There are some things missing, but for what it does, it does it really, really well.

I like :
FAST interface.  In and out faster then outlook any day.
Thread view of emails.  This is fantastic.  No more search for related messages and wait, and wait.
The spam filter is excellent, catching more then the yahoo one did 2 weeks ago.
Keyboard shortcuts.  They did them very well.

I dislike :
Contact's are very lacking.
I want to be able to add any message to an existing thread.

I would also put no client in the dislikes, but I did just find this link on Stefano Demiliani's blog.  Something worth checking out.  I also saw rumors of gmail being availibile in the google API.....  /me drools