I just got a gameboy SP since I was going was traveling a lot last month. And I also looked quickly into the flash cart thing. I read that the EZFA is good and simple, but lacks a lot of functionality the newer FA cards have.
The big issue is, if you want to play real GBA roms, you will have problems in the save game area. Since real GBA carts are always advancing (technically), the way games are saved varies from game to game. So the problem is how to do come up with a common save game feature for roms that work with your flash cart? The latest FA carts have these two small buttons on the cart that are used to help save games somehow (not sure how it works myself)...but this seems to be the latest way to do it (regardless of game). It also helps manage different game saves for different roms on the same cart.
Lastly, some RPG games use a realtime clock in the cart, but I think most flash carts now have this as well.
Let us know what you decided to get and post a review.