I sent a bunch of dummy mail to an outlook account to see how it handles having an inbox that's actually full. It's without a doubt the nicest webmail interface right now.

However I'm far too caught up in the GMail ecosystem -- Sparrow on Mac, Mail on Android -- to really care about a great webmail client. That being said with Google's acquisition of the Sparrow team I wouldn't be surprised if the race for nicest webmail was neck to neck soon.