Here is my not-very-objective review of MacOS 10.5 aka Leopard.
Installation was pretty painless - I uninstalled things like APE beforehand and after reading the various blogs where people had hanging installs, I am sure glad that I did take the precaution. It feels faster on my Quad-G5 PowerMac - things seem to respond faster. Visually - I'm going to have to change the default background - some of the annoying artifacts on the menu bar are from the background image. The work done to Mail - is excellent. Much improved responsiveness. The iChat improvements are very welcome - it can now be signed-in on multiple accounts.
I have not managed to get Time Machine working from using a FreeBSD fileserver, even when I have netatalk installed and operational - There has been some skectchy reports of workarounds but its not working here. I really don't want to have an external drive for each Mac for backup - I would prefer to be able to back them up centrally over the network.
With respect to software development - time will tell. There is already some indication that the assembler/linker may have some problems with some inline PowerPC assembler which prevents the build of the glib macport from completing successfully.