I've had zero problems with 10.4.4 and 10.4.3 on any of my G5's (no experience with Tiger on older Mac's). As long as you repair startup drive and the permissions, using Disk Utility and/or an application like Disk Warrior (recommended), before and after the upgrade you should be fine. Double check with any critical 3rd party hardware or software developers to be sure their products are compatible. I always backup/clone my current startup drive to another drive using an application like these:
That way, I can always go back to the working system if there's a big problem. In general, I'd say go for it. Every update, by just about every company, has a complaints from users about problems. What you don't see is the very large majority of users are having almost no problems with the updates. By doing the things I recommend above, you chance of having any serious issues is reduced even further.
I have installed Tiger on my G4 Powerbook twice and even after a clean install (erase and install) the computer slows to a crawl and eventually refuses to boot.
I'm at wits end. Any brilliant ideas out there?