IMO, the best thing to is of course back up EVERYTHING first! Then try doing an archive and install. Try it for a while and see what it does. If worse comes to worse, do an erase and install. Yes, you have to reinstall your apps, and restore your home folder, but its the best time to do it IMO. Gives you a clean start. Everyone's experience with Leopard is going to be different simply because everyone's setup is slightly different. No one Mac desktop is exactly the same. So if a friend of yours is having issues with Garageband or whatever, that doesn't necessarily mean you'll have issues. He/she may have something else installed that created the issue and its totally unrelated to what you have installed on your Mac.
I understand that its not always feasible to do an erase and install. It does make it more time consuming to upgrade to Leopard, but it will give you fresh start and eliminate any potential OS upgrade conflicts that may appear. But, at least there isn't 500 MB of OS X updates to install like there is with Tiger currently. So that will save some time right there.
If any 3rd party software conflicts occur then of course we all play the waiting game for them to be released. Developers have had plenty of time to get them out so I think its unacceptable to have to wait weeks on end to get them released.