As much as I love Thunderbird with IMAP, I'm finding that I'm making mistakes because of using Linux and Google Reader more and more. I use vim and both it and Google Reader use the vi-style commands of the j key for down, k key for up, etc. I can't find a way to re-map the keys in Thunderbird to do the same under Linux. Also, using Thunderbird means keeping two addressbooks instead of one.
So I'm now using gmail as my primary mail program, reserving Thunderbird for the times when I actually do need to work off the 'net. One of the things that I found useful was a way to Set Gmail as Default Mail Client in Ubuntu. This is working like a champ.