I just logged into my Yahoo account and got to try the beta of Yahoo's new email client. It's built by the same guys that wrote Oddpost. It's a huge improvement on the previous version client and I'd say it definitely gives Gmail a run for it's money. I just hope their spam blocking is improved, or should I say implemented. When you clicked the spam button in the old version all it did was put the sender in a block list. Oh yeah, that'll stop'em.
Gotta love the tabs and also it does RSS. It really feels like a normal web client. If you can't get used to Gmail's labels I would definitely give it a try.