I'll answer assuming we are talking hypotheticals here...
If signing up would let me download books to keep permanently, and you somehow would get books from major publishers, I might sign up for a month, and download whatever sounded in any way interesting.
If it's "read what you want as long as you are signed up with us", that is, no accessing the books when I stop paying... no. That would be worth at most $5 per month to me (which is six times as much as the local library charges).
If we're talking public domain/indie authors... ergh. Maybe. If you had
a lot of indie authors, maybe. But closer to $5 per month than $29.99. (Apologies to indie authors, but on sites where anyone can publish, there really are more books I couldn't stand to read for poor editing, requiring more time for digging... so I'd rather dig through free partial samples and give the
good indie authors money.)
Ah, the website jbcohen meant is
http://www.webscription.net/