I upgraded my system to OS X Leopard 10.5.5 via Software Update (not the combo updater), and there seem to be no overt problems with the upgrade. But there were a couple things to note.
This was a double-reboot upgrade, and it took some time to restart afterward probably due to whatever cleanup was being done. I noticed that the upper OS X menu bar was displaying incorrectly, in that the “bar” part of it was not visible - I could see only icons. After a while the problem resolved and the menu bar displayed normally. I am on a MacBook Pro with 2 GB memory, so there should not have been memory problems but you never know. After logging in, the Mail and iCal apps took a while to start with the status message “upgrading database”. It is best to log in, start your regular apps and just leave for an hour. After two day’s use, there seem to be no problems.