I'm a dedicated GMail user, but I think it's good to give credit where credit is due.
Autosave was definitely not first in GMail. Thunderbird also allowed messages to be sorted by thread a while back (which you could argue was a rough precursor to GMail's conversation view).
I really like Outlook. I switched to Gmail this year because I wanted free e-mail and Gmail is easier to configure. But, as a client, I enjoyed Outlook much more than I do Gmail.
