Culexus
The protection is hard. In today's world, where almost nobody thinks about someone else's hard work, and strives to distribute it illegally to all those who want it, such a program is simply worth its weight in gold.
The only negative is that protected ebooks with a large number of pictures and text (about 30-100 MB) open a bit slow for PCs and on smartphones. But this is due to the large size of the source file of course.
In general, I am satisfied with my choice. I have been using the service for more than six months.
Culexus