This past two weeks I've started to find myself abandoning Thunderbird for Gmail. I don't know why but I'm starting to find myself really bored with the whole three email accounts and a never-ending series of imap folders to fossick through to read all my email. Gmail is starting to grow on me. I think also, as the IMAP folders get more and more messages in them the imap server starts to groan under the weight a bit and accessing emails sometimes takes 30 secs or more. I wrote an email archiver to archive mail boxes for this very purpose ... but you know what, I like the immediacy of the gmail thing.
The only thing I don't like is the inability to synchronise contacts and stuff into my portable devices. But Thunderbird was never spectacular at that either. Outlook is the clear winner here, only because every portable device I have (both of them) automatically come with the required software and drivers to do it. That's an issue that Thunderbird ought to address (and calendar). If Tbird could synch with my mobile devices or had a really good calendar I'd be much more reluctant to abandon it.
I have 5 Gmail invites if anyone wants one. Email me, with 'gmail invite' in the subject.