A few weeks ago I ordered my Mac OS X Leopard at Mac at Campus and received it a few days later. As I'm writing on my diploma thesis at the moment I made a whole backup of everything and checked if it would be possible to continue my work on my linux machine. Not till then (call me a wimp ...) I started the installation of Leopard (I didn't choose update because I don't like the crap of old systems lying around) and it succeeded without any problem.
The best features of Mac OS X 10.5 are Spaces, Stacks and eventually Time Machine (didn't try it yet because my network storage is full and although there is a trick to backup on a network drive there's still the risk of data loss as it isn't supported officially). Spaces really gives some sense to multiple desktop environments (at least for me). Switching between two desktops is / seems faster than bringing an application to the foreground. So using it speeds up work enormously. Stacks is useful especially for the application folder. Okay, one may open an application via Spotlight (which btw. is faster than in Tiger) but I never got used to it.
Todays update to 10.5.1 didn't bring much new for me but only a laughter about how they reacted to the (justified) attacks against their firewall: They just renamed the Option "Block All" to "Allow Only essential services". Yeah, it's not a bug, it's a feature ;-)