My two cents on the DS-X.
So I bought this as my first flash cart. Never updated the firmware, and I have never had any problems with the games that I have played. Multicart play works with the most of the better multiplayer games such as Mario Kart DS, DKR, EBA, etc, and that's specifically what I like about it. Knowing someone with an R4DS, it kinda sucks not being able to play them while in a boring Biology lecture.
Loading times are not bad... maybe it's because I haven't used another flash cart, but I can wait out the load like nothing. The price maybe steep but it feels like a cheap buy actually after I started using it.
Only problem is the 4Gb size. I have EBA, Ouendan, Mario Kart DS, and Metroid always in my cartridge because of mad urges to play them at random times. That leaves only about 2-3 more games. Plus when Pokemon DP come out that's going to be there all the time too =/