R4s are still perfectly good, tho some of the '1337' piraters will tell you otherwise, as the R4 hasn't had any firmware updates in a while, so some recent games with advanced copy protection don't work. however, for homebrew it's fine and always will be. and even for piracy, there are "cheats" and patches that will bypass this copy protection.
my favourite multiplayer game is tetris ds. this game is amazingly addictive when played head to head, and it's awesome online as well. however, this game is so hard to find these days that people resort to importing japanese versions (which are plentiful in japan), tho i suppose if you have an R4 or whatever that wouldn't be a problem for you.