I use Gmail. And I use it for everything.
However, I need to use Thunderbird for a lot of reasons, the big one is that I can’t do PGP email signing from Gmail. So, in order to use Thunderbird, I need to have a few things in place.
That’s it. I install Enigmail and Zindus, hook them up to their respective accounts, and I’m set. It’s simple as that. And now that I’m using openbox as my window manager, I can have Thunderbird running all the time without slowing down my netbook much. In fact, once I get my second GB of RAM, I probably won’t notice Thunderbird’s drain at all.