I like it very much. Rated 5 out of 5 stars
It can has possibilities that there's no reader has because it is runned in native html. It can works with Firefox addons e.g searching, dictionary, encyclopedia tools. In future maybe someone will write addon that could fill forms, comments pdfs. I have bookmarklet that can fast search double clicked word in dictionary. The one works great with this. I guess there is no pdf viewer with such possibilities. It has a potential!
PS Unfortunately for the moment it doesn't work with some pdfs but developers works at this.