R4 is very cool, has screen skinning capability, has a cool menu system that will let you choose games, music, video, boot a Slot-2 cart.
My son has an R4 w/ a 2GB MicroSD card in it. I have grabbed roms copies of the games I've bought for him over the last few years (I know there's a vague question of whether or not they're legal unless I dumped them myself...whatever, I bought the damned games, most for full price, in my crazy, hippy world, that's plenty). I've also grabbed a couple of games that I didn't buy first to see how they were before making the decision to buy them. If we decide he doesn't like the game, I delete it and we move on.
The fact is most games on the DS are trash, and there's no really convenient way to find that out without either buying them or downloading the roms to try them out. We own around 30 DS games now, and 10 or so of those were bought after downloading the rom and liking it enough to purchase. I think we use the R4 card in a legitimate, honest way, I know most people don't, but I can't control those people, I can only vouch for my own approach to using one.