It's an interesting question from Rands In Repose, aimed specifically at gMail:
The question is, "Does Google have good UI?"
Personally I love it. There are few features missing, like the ability to save a draft message, but on the whole the simplicity and clean interface make for a pleasant web-based email system. But really, as a programmer I'm the first to admit that I'm not the best person at UI design. Oh I reckon I'm better than some, but I can't help but think that the user will figure it out, sadly often not the case...