As far as I can see, a PPC Mac is still viable in almost every way except access to the internet. For that, my (limited) experience of it is that it is distinctly compromised by modern sites with multiple redirects, scripting and ads.
However, particularly when using period-appropriate software, one of these systems is potentially as good as it ever was. It can still be used for email, office software such as Word, iMovie, iTunes, etc. Even Photoshop Elements. All the software that used to run on these systems still does, and still works much as it did.
The challenge is in finding the limits - which versions of software are good and which are not - but I certainly get good and regular use out of my 2003 G4 iMac. And it sure is great getting reacquainted with otherwise long-forgotten applications such as iMovie HD6!