I modded my GC a while back, and have had no luck with playing anything (homebrew, ISOs, whatever). Original discs still play, and the modchip definitely works (it has a menu that can be brought up to confirm it's working) but anything I've tried to burn has failed.
I attempted different brands of mini-DVD-R, I tried adjusting the voltage pot on the optical drive, and all the usual recommendations. Nothing worked. I looked around for cases that allowed regular DVD-R, but they were crazy expensive (more than I paid for the GC in the first place!).
Some time later I soft-modded a Wii (you can buy these for $30 second hand here in Australia - probably much cheaper than that in the US). With DIOS MIOS (slightly tricky install) I can play GC ISO images straight off a USB hard disk without the need to burn anything. If you don't want to go through that pain, you can just burn them to regular DVD-R and play them quite easily.
I can't think of a reason why anyone would want to go through the pain of modding a GC - perhaps only if you were keen on playing import titles on a domestic GC (even then, I'd probably just buy an import GC to match if I was really after that level of collected item). A modded Wii is a much easier path, IMHO.