I had a look at it after following a link in someone’s email signature and set up an account. Free first, then member, now subscriber.

At first, I hardly used it; at one point, I forwarded my LiveJournal mail there (including comment notifications) while I was on a training course where we had web access, and since then, I’ve used it quite a bit and rather like it.

I especially like some parts of the interface I haven’t seen elsewhere, such as the keyboard navigation (e.g. delete-and-go-to-newer-message: Ctrl+Space, ‘d’, ‘,’ — something I can’t do in “one” step on Gmail, for example). Or even something simple as selecting a range of messages by clicking on one and shift-clicking on the last (GMX doesn’t do this, for example).

I think it deserves to be more widely-known, but when I hear about webmail, it’s nearly always only Hotmail/MSN, Yahoo, and Gmail.