I've got an EZFlash 4 for slot 2 (got an original DS, not a Lite, otherwise I would've gotten the Deluxe edition). It's pretty nice once you figure out how to change the skin and the terrible Engrish and everything. Would've preferred something that supported Pogoshell, but whatever. It's the easiest and cheapest card around, and it took me quite a while to find a slot 2 card that actually ran GBA games Go figure. It seems to run DS stuff decently as well, but I don't use it for that very often. Only problems I've had with it is it doesn't like to run two-player stuff sometimes.
For slot 1, I got a DSX, 512MB. Everyone seems to be ranting and raving about the R4 these days, but when I got mine, it had the best reviews out there (and it had no Engrish, a huge selling point for me). It certainly could be better, and their customer support is terrible, but whatever...I have very few problems with games, with minimal dinking around.
It's funny, I was really excited about getting flash carts so I could run homebrew...and it turns out that most of the good homebrew is emulators =/ Still nice to be able to play a lot of stuff on the go though, and I love that I can have all my GBA games in one convenient cart...not to mention, I'm a really big fan of try-before-you-buy =)
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I'll brave the storm to come, for it surely looks like rain...