I own the GBA Movie Player. The thing now sticks out too far in my DS Lite, but that is expected with any large GBA 'cartridge' now.
This is a fun device. Its movie player is fun to play with, but if you want a portable movie player, get something else. For smaller TV shows and clips, this can work well enough. For movies, no. Also, the longer the clip, the more of a chance the sound and video will go out of sync (since the two are seperate files). But it is no big deal.
The music player is iffy since the music is going through the GBA's sound. For better sound quality, you are going to have to use Moonshell and hack the DS slot (worth it).
Text reading? It moves too slow. You aren't going to be reading any books on this thing. Picture viewing also is silly on it.
The true gold of the GBA Movie player is that you can run the NES and Gameboy emulators. If Nintendo wants to print more money, it could make a portable version of the Virtual Console. I bet the third incarnation of the DS will have that capability (with the emulation built into the firmware).
I reccomend the GBA Movie Player. While its capabilities are not mindblowing, they are fun, and it is worth it for its cheap price. If they have plans for a DS Movie Player, I would wait for that though. The flaw of the GBA Movie Player is that everything must be run through the GBA part, so it cannot utilize the DS's superior sound unless you do some further tweaking.
I will add that after I imported my GBA Movie Player, other people got envious of it and bought their own. Just by showing this device to people, I have had like a dozen people buy it soon after. Take that for what you will.