One of the better cards (got a Golden Award at GBA Temp) and in stock at DealExtreme at the moment.
100% ROM compatibility
100% Download Play compatibility
Soft Reset works 100%
Cheat system works
Very fast and responsive menus
8 languages supported
Open Source (only DS card company to do this, though nothing has come of it)
Negatives are you have to compile the cheat files yourself, though this just means you don't have to wait for the manufacturer to release them in my opinion and that's if you want to use action replay codes at all. Also the build of the card is reportedly not as sturdy as it could be but if it works....
Update: Gone down in price from approx £16.18 / $31.90 to approx £13.90 / $27.57, updated title and price.