I wouldn't suggest using the GBA4iOS app if you don't own copies of the games (to stay legal and all) but it works really well for a GBA/GBC emulator. The on screen controller is tough to get accustomed to on a large iPad screen though. I'm hoping one of the new iOS 7 controllers coming out this year turn out to be good. So far, I have yet to use a controller for the iPad that actually works well. .
The app is not available on the app store for obvious reasons. You have to custom install it at your own risk. No jailbreak is required.
I loaded up Dragon Warrior III to test the app...one of the best Game Boy Color games!