I am using the R4i RTS with the N3DS, no problems with it so far. Only thing is that you'll need to update the kernel once in a while and depending on the firmware of the N3DS, you might to flash the flashcard as well (note: this is different from updating the kernel)
As for the games, forget about N3DS games, this only works with the Gateway card. However, the card is still buggy and only works for N3DS till version 6.1.0.XX (iirc).
Regarding GBA games, it is possible on a R4 card, but once again, it is very laggy. Most GBA games tend to crash (when sound is on) and it requires a separate program on your flashcard. If you want to know more, check here for more information
DSTwo however, is able to play GBA games, I quite certain Rey will be more than happy to elaborate on this :p