I recall seeing it as sluggish and awkward when I finally decided to check it, abusing Ajax. Other web interfaces (that I tried) were more lightweight and predictable, and regular mail clients I found to be a much better UX. So I've mostly attributed Gmail popularity to Google promoting it, possibly using that invite-only policy to make the users to spread the invitations. It sounds like at least some users actually liked it, but I don't think it's a clear example of a good service by Google.