I like thunderbird, and have used it extensively for years multiplatform. However, I feel that it is the unloved stepchild of any hair colour in the Mozilla family. I would like to see something that continues refinement to something that is nigh on perfect (I still suspect memory consumption could be easily improved with Thunderbird, althought checking it now I am pleasently surprised. Maybe I am being harsh. ).
I am seriously concerned about the future of Thunderbird. As the Mozilla-based backend continues to migrate in inexplicable ways, they'll have to expend more and more effort shoehorning it into the client. And they're finding themselves with fewer and fewer staff, rather than the more that they need. If TBird breaks, there really isn't a usable replacement for it. The calendaring functionality (at least when tying into the Google Calendar we use at work) is already severely broken, I don't see them being able to fix that with the resources they have now.
Throwing money at them will only go a little ways towards fixing their problems; what they need is development staff, and they need it quickly. Just dread the thought I'd have to resort to web-based email (*ugh*, *spit*). Would definitely be cancelling a majority of my mail lists at that point.