Not sure why you are getting down-voted. I have 2 MBPs. One is s ~3 year old running 10.5 and his baby sister running 10.6.
This is entirely subjective per my experience:
- the 10.5 'feels' more responsive.
- the beachball is a very frequent event in the 10.6, specially Safari, sometimes even when switching tabs which is quite annoying.
- I haven't rebooted the old MBP in months. Sitting pretty on my desk. The new one has frozen few times and occasionally I feel the need to put it out of its misery myself.
There's no such thing as a trivial OS upgrade, and so long as 10.5 is supported and runs the stuff I need to run, I don't have a compelling reason to go to the trouble of upgrading.
You'll have to excuse me, but you're wrong. I mean, sure, it is an OS upgrade. But there's no real "trouble" (it takes about 20 minutes and a few clicks) and there's definitely compelling reasons to upgrade (including running 10.6 specific software). One pressing reason to upgrade: 64bit. Another one: cocoa finder. Also, faster and smaller.
Seriously, I know people who have a reason not to upgrade. You don't seem to be one of those people. Those people are on worse operating systems than you are.
OK, tell you what: you come over to my place, run the upgrade, then fix all my dev stuff (combination of macports and non-macports) that'll break. Seriously, I've done this before and I know what's gonna be involved. But if you want to sit back and tell me how little "trouble" I'll have, you can do it for me.