One big snag at this point - tests with the new MacBook Pros indicate that you can't currently boot a Mac from a Thunderbolt-connected device. Thunderbolt does work for Target Disk Mode, and Apple does plan to add the ability to boot via Thunderbolt in the future, but it's not there yet. Once it is available, performance via Thunderbolt will exceed SATA3 on devices that completely saturate SATA3 (RAID 0 SSDs, for instance). Of course, it's probably only a matter of time before internal drives are on Thunderbolt, as well.
I've been booting my 2009 i7 iMac from a FW800 Intel X25m 80GB SSD since the day I got it. Startup times are a bit faster than the internal WD Caviar Black drive, and application launch is much smoother. My user folder is on the internal disk, so there is no contention between reading OS files and user files simultaneously, as there would be if I was booting from the internal disk. I definitely recommend it - you don't need to buy an expensive SSD, since any decent one will saturate FW800, and you don't need a ton of space if you put your user folder on the internal drive. My SSD is only half full, and that's with 10GB of Xcode 4 installed.