The .emu series are the best if you take into account usability, compatibility and performance. Obviously on the face of things taking code and violating GPL is wrong but I think the fee is well worth the support you get, in fact I would buy one of Robet Broglia's emulators specifically because of the support.
Android is hella fragmented and that guy goes to great lengths to ensure the app you bought runs on your device with the absolute minimum effort on your part, he sent me experimental builds of emulators to ensure compatibility and was more than helpful. To do the same with RetroArch requires extensive testing and messing around with manual refresh rate settings.
Robert shows a real understanding of his market, people who for the most part want to download something and have it working with minimal fuss. To do this with Android requires a lot of work and communicating with users, basically a huge chunk of admin which isn't going to end anythime soon with the constant influx of new devices. Making emulators for Android is a pretty big undertaking considering your target market
I read elsewhere that RetroArch don't accept monetary donations but do accept hardware (they mentioned an iPad2 as a possible gift!), seems like a strange move as I'd happily donate if it made the app more user friendly and not so much like PC emulation. Portable emulation is different, I'm much less inclined to sit there messing about with settings and testing than if I was sat at a PC.
I also find it strange that they are so against supporting frameskip (calling it unacceptable for emulation) yet support virtual touch controls which for lots of people completely ruin gaming. Give me 0-2 frameskip and a real pad over a solid 60fps and crappy touch controls any day of the week!