I've bought new WiFi
card yesterday specially for running those NDS demos. Hope we will see more of them coming out soon and not only Japanese demos.
Some impressions:
Installation of PCMCIA card was fast and painless, I made some .bat files for transmitting all od the demos and was plaining Meteos demo in about 10 minutes after opening WiFi card's box. At first I thought I'm screwed because my EZFA 256 Mbit Flash cartridge isn't flashable in DS (lack of necessary slot to connect USB cable and I've already sold my GBA & GBA SP). I've inserted F-Zero GBA cart in my NDS, hoping it will work. Actually, it's possible to download and play those demos even without any GBA cartrudge inserted. One more thing I noticed - you need to remove/insert PCMICIA card after transmitting each demo or else you get some device_failed_to_send_blablabla error.
About the demos:
Meteos demo was simply awesome and fast, it reminded me of Tetris Attack. Anyway, it was on my preorder list even before trying out the demo.
Polarium: completed the tutorial, I like the idea and all, but I can't seem myself paying more than $20 for it...
Table Tennis tech demo - overall simplistic, even for demonstration of DS touch screen.
Trauma Center: Under the Knife: playable even without Japanese knowledge (nurse and doctor giving you on-screen hints). Completed both operations, definitely will play it again to improve my score. US version is already on my preorder list.
Submarine tech demo: Nintendo, please release commercial demo using the engine and psychics of the demo, throw in some story and underwater torpedo battles!
Goemon: demo was too short to decide about purchasing the retail version, couldn't manage to beat the big yellow 'bot in two tries.
Training for adults: Work your brain: tried matchematics quiz, it's a pity we won't be seeing US version of this "game"

It would be instant purchase if they would keep the JPN pricing.
Electroplankton: I'm not a big fan of music mixer software, yet this demo shows the audio capabilities of NDS.