I waited until 10.5.2 came out before getting the Leopard update for my old CoreDuo 20" iMac and my MacBook Pro both running Tiger. I wiped the iMac20 and installed Leopard fresh, but I upgraded the MacBook Pro because it is my work machine and I didn't want to mess with the system. A few days later my 24" iMac turned up; so I had iMac24 with preinstalled Leopard, iMac20 with a fresh install of Leopard and my MacBook Pro with upgraded Leopard.
Upgrading is a mistake. I highly recommend telling your Leopard update DVD to wipe everything off the disk and install fresh. My upgraded MacBook Pro was running like a police officer in love with jelly doughnuts; which is more of a waddle than running. The system was constantly accessing the hard drive, I had regular crashes in parallels and a number of kernel panics. There is something very deflating about productively using software only to have it all go away in a blink. I thought I had left that behind when I escaped the foul grip of Microsoft's Windows. This situation peaked at three incidents in one day, where I just spat the dummy.
Both my iMacs were purring along with their modern feline titled operating system, why not the MacBook Pro?! So I bit the bullet. After a session of Time Machine I reformatted the MacBook Pro with a fresh new copy of Leopard. Then I copied across my document files, pictures, etc form Time Machine. And lo, the system is better than it has ever been, the sky is bluer, grass is greener and the birds are singing.
I am unsure what exactly the problem was. I did have a lot of extraneous pieces of software that I was using or testing, but everything was functioning fine in Tiger and changed to a pig when I upgraded to Leopard. So if you don't want Pig-Leopard you have to wipe the slate; tell your Leopard install process not to upgrade but to wipe that ugly disk clean of the previous beast and grow your Leopard from scratch.