As bit shifter said, the EMS USB carts have some crazy bank switching issues. From what I can tell, when you plug it in to your computer to put roms on it, it messes up the bank switcher. Possibly gives the capacitor too much charge? It wears off eventually, but there was a 'break in' period where it just didn't really work at all. So they're great, but just don't really plan on switching the roms out a lot.
Also, one thing about them that hasn't been mentioned is that they can only hold one sav at a time. It also seems that roms in bank one overwrite the bank two savs. I have lsdj in bank one, and if I fire up a bank two rom and save it doesn't delete my lsdj sav, but it loads the sav up into lsdj which can produce some crazy stuff. All I do is reload the song within LSDJ.
I agree with bit shifter, the smartboy carts are probably a better bet, as they come with a transferrer. Good luck getting one from Japan in any form of a timely manner though!