We access Gmail over IMAP, and we specifically optimize for their servers (ie: we don't bother supporting the full range of the IMAP protocol, and we take advantage of Gmail specificities)
This allows the client to feel fast, even though actions that have to sync with Gmail, like deleting or archiving something aren't really faster or slower than with the web interface. But they happen in the background, so it feels snappier.
We will be releasing a private alpha in a few days. It's still very early, though. You can read / delete / archive / reply to emails in your inbox, and that's all for the moment.
Drop me a mail if you want to try it out, or sign-up for the beta at http://betterinbox.com (The beta will be released later, in a few months I think)