Problem with Sony is that they're pretty anal about home-brew, if you ask me it's mainly because some idiots out there use ISO loader programs to load up illegal copies of ripped UMD games, where-as the rest of us play some of the fun home-made programs
The UMD loaders don't make a difference. It's not possible to run them with the GTA exploit, just the emulators and other goodies, but Sony still patched it up with the latest firmware release. Pure a-hole mentality. Try to get a psp with 2.00 firmware or below, the 2.00 is the latest version that can be downgraded, if you can't then it's better to just forget about it. Newer psp games won't work with 1.5 as-is but the homebrew scene is very active producing patches for those games to run on 1.5.
Is it possible to play GBA games on the PSP yet?
Still way too slow and buggy.