Today I invoke the Software Update manually and there are a few updates, including iTunes 7.5, QuickTime, and of course the Leopard update 10.5.1. Installation took around 15 minutes. Being a new Mac user, I cannot spot any difference between this update but it might be more stable?
Also I found a very strange problem with Leopard recently where when I boot up my Mac (normally I use hibernate mode 5 - similar to Windows Hibernate) and after everything is booted correctly, when I click on any application in my dock, the application icon kept jumping and the application never open. I have tried Terminal, Activity Monitor and even tried to force quit some of them but failed. Ended up have to force a restart by holding the power button! I don't really expect this to happen to a Mac, which contradicts what I heard from others, where Mac OS X seems to be the most stable OS!
Having two incidents since I bought my Mac, one being installation of Parallels and now cannot open any application (after force reboot of Mac, it looks fine), it kind of makes me wonder if all the reviews that I heard are true?