You've probably already looked at the "Membership & Subscriptions" section of the Joomla! Extension Directory - if not have a browse.
We've used several on Joomla! for our own websites (AEC, Ambra Subs and RSMembership!) but over the last few months we've been recommending Akeeba Subscriptions in conjunction with Akeeba Release System. We're also migrating our sites the the Akeeba solutions, in fact we've only got one left (a software site).
As a background we used AEC most extensively after working our way through the Ambra and RS products and hitting limitation with them. We've stopped using AEC as it's far to complicated for the average user (ie. our typical client) and frankly was getting unwieldily even for us.
One of the key contributors after easy of use and setup was the good documentation not only for users but for creating plugins for the Akeeba Subs software (in our case we created a plugin for an Australia payment gateway very easily, much easier in fact that we did for Virtuemart - which is poorly documented).
The Akeeba system is used on their own website to sell their Joomla! backup solution and from our experience to date is both robust and flexible. We've only used in on Joomla! 2.5 so we've been able to take full advantage of the new ACL in conjunction with Akeeba Subs.