The app is really similar and is very limited in terms of functionality. The big difference is being able to read books in epub, although the app doesn't keep track of your place or manage a library or anything fancy like that. Using the app, I cruised through ~90 pages of "It's Complicated" by Danah Boyd (http://www.danah.org/itscomplicated/) in about 15 minutes. Afterwards, I had a strange brain sensation bordering on headache, but felt like my comprehension was high.

The big limiting factor at the moment for both the bookmarklet and the app is that they do a poor job making specially formatted stuff readable (like numbered lists) and seem to not manage parentheses and other punctuation very well.