The SP+ is so amazing that it's worth holding out for. I put Mario Kart GBA in there last night and was like :-O because I never realized just how colorful that game could be. Seriously, with that awesome new screen, the games simply don't seem as out of date as GBA games were starting to.
Kuru Kururin Paradise, Metroid and Castlevania prompted a similar response. You simply have no idea how vibrant those games are until you see them lit up properly.
Famicom Mini SMB had a little red bleeding though.
I guess a Micro has the same effect, but that screen's too small for its own good.
I've used GBAPlayer plenty of times too, but that mostly brings out the games' lo-res flaws.