I was cleaning out some of my handheld stuff the other day and ran across these after being hidden in a box for nearly a year. A while back I ordered some DS flash cards from a wholesaler and they ended up sending me the wrong thing, they were supposed to send an R4i clone that was compatible with the DSi and 3DS and instead sent these, which are only compatible with the original DS or DS Lite models.
I've owned one for a very long time and have no complaints about them. They support all the usual homebrew stuff along with wireless play, Action Replay codes, etc. They're brand new and still in the blister packs. $5 USD for the cart or if you need a 4g sd card it's $10. I'm not making anything off this, just trying to get my money back.
If nothing else it's extremely cool to use to back up your GBA game saves from your real carts if you want to replace the batteries, or you can transfer your GBA saves from emulators or websites to your real carts.
I've got a Flash-something-or-other for my GBA SP and love it. Probably one of the best purchases I've ever made(it was only bought a few years ago, so the GBA was well-dead)...the GBA really did keep 2D gaming alive just a bit longer. I wouldn't mind trying out some DS games on the 3DS, but I just find that virtually everyone I get a hold of just makes me sick to my stomach(the DS seems like a portable Saturn or something). Still, for someone with a DS, well someone with a DS who likes it, it's a good buy.