I've been experiencing the SAME issues with Mail and IMAP Gmail.
It takes forever to download new mail -- and this is even when new mail is actually downloaded. I've had a number of messages not come through. These messages are NOT spam.
Everything is FAST in Thunderbird.
Sucks... I want to use Mail, but it's very very slow and un-reliable.
Thunderbird is awesome -- very feature-rich, but ya, doesn't look as nice as Mail and it's not as integrated with the OS. One thing I *hate* about Mail is the attachments. If I attached a file, it goes inline, whereas Thunderbird keeps it separate from the text. Small thing, but annoying.
However -- Thunderbird has NO integration with Address Book. Another disadvantage. Ugh... this is a PITA.