Subscriptions worry me,
You'd need really tight DRM to make it work, and it also feels like one step closer to removal of peoples ownership of an originally bought copy of someones work.
Instead of owning a book, your renting it. Instead of being able to lend your book to a friend, he needs to rent his own copy.
I'm sure it's going to be successfull, we've gone from economical ownership of alot of products over the years to merely renting them. With somewhat interesting consequences for example, local households has effectively trippled their debt over the last 20 years.
It reminds me very much of this intriguing RSA animate:
http://www.youtube.com/watch?v=qOP2V_np2c0
In terms of convenince, subscription models are very likely an improvement. It'll give us greater access to litterature at the expensive of genuine ownership. I'm naturally talking a for rent model here, as bulk buying clubs has already been tried without too much success.