I've been using Yahoomail's Beta version for a good number of months now (you can go to account settings to try it out- if it doesn't appear, switch country settings to Ireland/UK, and then select it).
It basically has the look and feel of a mail client like Outlook, with drag and drop capability added in, and tabbed windows for opening e-mails, searches, inbox and the like.
I recently started using my gMail account for a show, and though I'm intrigued to see what the big deal is, it freaks me out a little. I just got on top of being selective of what stays in my inbox, what gets archived, what gets deleted, for my yahoo account. This conversation business in gMail, I do not find helpful. I like being able to associate a single e-mail with a certain task, or as backup info for a certain task. When everything collapses together in a "conversation" type thread, it makes it slightly more difficult to see what the key new information is, and then decide what to do about it/where to put it.
Comparing the interfaces of yahoo and gmail, yahoo looks a lot nicer. Plus, if you are in the middle of composing an e-mail and you want to check out, say, contact info or another e-mail from your inbox, in yahoo it'll just open another tab. You won't have to navigate away from what you're composing.
I haven't tried RSS feeds yet in gMail, but again, I am happy w/ where Yahoomail beta puts me on this. We'll see.