OS X Leopard
I have 4 computers (1 laptop and 3 desktops) packed away - I now do everything on one fully loaded MacBook: for medical reasons(*), I either need to have my legs elevated or be walking - sitting at a desk for more than a few minutes is not a good idea. Fortunately, I can still hike for up to 3 or 4 hours at a time :-) Anyway, having everything in one laptop works best for me.
Anyway, since the work and research part of my life is bound up with one nicely configured laptop, I was hesitant to change anything, but after burning 4 backup DVD-Rs and cloning my laptop's disk drive to an external disk, I did a fresh install last night and restored my user files from the cloned disk.
It took a few hours to get all the tools working that I use (and I am probably still not there yet), but the time was well spent. I think the Time Machine idea is awesome - you really need to try it to appreciate it. I plan on alternating between two external disks, keeping one at a friends house for off site backup.
I am still deciding if the Finder changes are all that great. The icons don't seem as distinctive as the old ones. I do like the document automatic preview options.
The Java JVM support (version 5) performs well, and the NetBeans IDE (used only for Ruby and Ruby Rails development) seems more responsive. Ruby is installed as a shared library framework so if you should run many simultaneous Ruby processes, some real memory would be saved (like Apple does for multiple JVMs running). I was going to do my own Ruby install and decided against it.
Leopard seems very much faster than Tiger but that may be partially because I wiped the disk and did a clean install (the ultimate disk defragmentaton :-)
(*) A bit of advice: last year I was working really long hours sitting at my desk without getting up often enough - that lead to deep vein thrombosis and 2 pulmonary embolisms. Set a timer to remind yourself to get up every 30 minutes or so!