The Netflix 3DS app is available now, and I had a chance to try it out.
Once launched, you're asked to activate your 3DS via code with your computer (but from what I hear you can also do it via the 3DS web browser at
http://netflix.com/nintendo3ds and logging in, but I haven't tried it myself). Once activated, you're brought to a Wii-like menu system. From my experience with the Wii app, the 3DS version is slow in comparison. Tried out Iron Man 2, but it wouldn't load. Had to do a hard reset because nothing else worked. Not even the Home button worked. It appeared that others I've talked with who tried it were having similar problems. After rebooting my 3DS and relaunching the app, the interface seemed to run a bit faster (but still not as fast as the Wii version). Iron Man 2 loaded, though initial buffering wasn't as fast.
Playback of the movie was surprisingly smooth. A little bit lower quality, but it is not such that it's a problem. Volume is decent (when compared to the downloadable trailers from the eShop). They do plan on adding support for 3D movies in the future, so I could at least assume that based on that, 2D movie quality could be improved, as 3D movies are kinda like 2 2D video streams running together.
Overall, it's a good app. I did forget to mention that my Wii is connected online via network cable rather than WiFi, so that could explain why the 3DS version feels slow to me. Perhaps they could improve on that as well.