I have a few of these, however they are pretty limited on how many cheats you can hold. First issue is the fact that they use the xml format, which is the most clunkiest and most space consuming cheat database to use. Second issue is the fact they are limited to 2.5MB of cheats, adding too much causes the software to tell you that there isn't enough space for more. Knowing how my databases are these days, you will have fun cutting down a whole cheat list down to a couple or so codes.
The Action Replay DS devices are indeed a nostalgic piece of hardware that regardless of the issues, I still enjoyed. It was a great way to use cheats on physical game carts rather than having to emulate everything to do that. I still think they are pretty cool to use. Flashcarts can use NitroHax too which is basically the equivalent to the Action Replay DS just in digital form, you can still use physical game carts. Flashcarts really blew the devices away though due to the fact that there isn't a limitation on how big the cheat database has to be, plus those use a more optimal database versus the clunky xml mess.