The Compat isn't that bad, but it's not 100% unlike other carts
http://www.gbarl.it/index.php?showtopic=15642
the Super Card range has a rather poor compat
The Slot1 flash cards have the best compat then the next up are the M3 and G6 flash cards with passme type slot 1 device ( M3 = GBA Movie Player G6 = by the same people but use internal memory instead of memory cards )
The olnly issues the slot1 cards seem to have is for single card download play
P.S. Just went for a M3 myself. Original sized SD version and I'll only be using it for GBA. I wasn't bothered about the size because I plan to buy another GBA SP at some point and I own a GameBoy Player that the fitted DS lite versions of the flash card would not fit into my GameBoy Player and the GBA SP I plan to buy