I've submitted two bug reports about minor annoyances in the interface, and both got a "we'll get around to reading your report eventually" form response. I think that's another reason they've stopped/slowed giving out invites (besides people selling them).
I have about 8-10 mailing lists that I have my gmail account subscribed to. I get about 30 new "conversations" a day (probably 70-80 new messages). I like the way the conversations are handled better then the threading in mozilla mail or evolution, so I'll continue to use it for the mailing lists. However, I like the imap/webmail support I have with my current e-mail, so I'll stick with that for my regular e-mail.
I'm not impressed at all with the spam filtering though. I'd say about 50% of spam gets through, and I get 2-3 false positives a day.