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...