I've seen no data that suggests any of them is long-term viable.
Doesn't mean it doesn't exist but so far nobody is bragging about number of users or growth rate. It has been suggested they are profitable but only because a lot of their members don't actually use their services. (The claim I saw said less than 1 book a month on average.)
Kinda like gym club memberships.
That might be viable in the short term. Long term?
I dunno.
None of them appeals to me, although Juli Monroe of Teleread had this to say back in august:
http://www.teleread.com/scribd/almos...anged-reading/