I just spent a couple of hours playing around with Flipnote Studio. The program is surprisingly rich in terms of drawing options. There is an accompanying Youtube-like service called Flipnote Hatena, hosted by Hatena, a Japanese internet service company; The flipnotes can be accessed from the DSi through the Flipnote client or via the web through flipnote.hatena.com. Users can draw and make their own flipbooks on their DSi or DSiXL, however posting flipnotes, commenting on notes, liking/favoriting notes require an account which works on both the website and on the DSi.
Note also that the flipnote.hatena.com website is meant to funnel viewers to the flipnote creations. The creation of the notes (drawing, coloring, animating) is exclusively done via the DSi. The integration of Flipnote as a full featured service for DS users is quite impressive indeed and it's all FREE!
For me screen size and comfy pen sized stylus of the DSiXL makes all the difference here. I've had flipnote on the DSi for a few years but always found it too difficult to draw and never got into it. Anyways, here is my first attempt at doing a flipbook animation. (and yes, anyone can view your DSi flipbooks on the web and they can be shared and embedded in blogs as well!)