They are outstanding for pretty much any PPC Mac. Two concerns are power and heat. These cards require an additional connection to the power supply so a marginal power supply may struggle to provide power. Also, these suckers put out a lot of heat, so you may need to take special measures to dissipate the heat so you can avoid crashes and freezes.
Just as a note, they are the best cards in any PPC Mac where they will work, but there are a lot of what-ifs and caveats to that statement
1. Of course, nothing before the Sawtooth G4 even has AGP, so they're off limits for that.
2. The X800 won't work in a 2x AGP slot, so the Sawtooth, Gigabit Ethernet, and Cube(assuming you could shoehorn one in) are off limits
3. They are iffy in a lot of 4x G4s-Erik and I both have had serious issues getting these cards(or at least the flashed versions) to work in Quicksilvers. I've never been successful in getting one to work in a Digital Audio either.
4. They are generally reliable in MDDs, although they do need to be taped
5. They are great and plug-and-play in G5s up through the late '05s
6. They are off limits in the late '05 G5, since those use PCIe. It's a shame, too, as this card benches better than high end FX4500 that was BTO in the '05 G5s.