In short Thunderbird mostly works the same as Evolution, but with fewer bugs and annoyances. The big difference - the one feature that had me thinking of switching - is that Lightning, the Thunderbird calendar, can seamlessly synchronize both ways with Google Calendar. Other benefits are quite minor; for instance, with Thunderbird you can always jump directly to the next unread message even when it's in a different mail folder, whereas Evolution requires you to select the folder manually. You can undo message deletion with the general "undo" command, while in Evolution you need to view your deleted messages, find and select your email, then undelete it. Small things, true, but they add up for a much smoother overall impression.
There are a few areas where Evolution is clearly superior. The automatic account set-up worked better with Evolution; I had to manually re-edit my mail accounts after setting up Thunderbird in order to get them to work, and it never gave me any useful feedback as to what wasn't working (or that one account wasn't working at all). Also, the manual mail check button doesn't seem to work consistently with Thunderbird, and there is no visual feedback when it fails. Overall, though, Thunderbird is an improvement, if an incremental one.
A final note of caution: actually moving my accounts, address book and my archived email from Evolution to Thunderbird was a bit messy and error-prone. You need to set up your accounts manually, add a temporary account to import your old email, and you need to export your address book then use an external converter to get it into a format Thunderbird can read. It's certainly doable and there's step-by-step guides out there but it's still nothing I would do lightly. I expect that Ubuntu will will have an import function to automate the move from Evolution for the next version. If you're not in a hurry to switch I would suggest you wait for that.
#1 I've been using it as my OS of choice since my days as a computer science student and I'm happy with it. But that's not the sole reason; nowadays Linux is the de-facto default operating system for much of neuroinformatics, computational neuroscience and high-performance computing. Simulators, languages and related software tend to run primarily on Linux, and most computing clusters and supercomputers today use Linux adapted for their particular hardware.