I think they are fine though I personally wouldn't get one, much like
@Drathnoxis I think something like the Everdrive is just more convenient, it allows you to circumvent all the emulation issues with the additional convenience that you don't need to go to your bookshelf to fetch the correct cartridge because you can have all the games on the same one, this is particularly better for portable devices like the Game Boy and DS lines as it also saves you the convenience of having to carry those tiny cartridges that could easily fall and be damaged or lost, which is fantastic, more convenient and less risk of damaging possibly collectible and expensive games.
That being said I do understand the appeal of reproduction carts as it does fill in for that compulsion to just have the thing on your shelf, I still wouldn't get one though.