I have been using S2Member pro for a three years and I have to say it is outstanding! The free version is great too, but I like a couple added features for the paid version (like coupon codes) and it is well worth the little they charge.
You pay one fee and get lifetime support and updates! Support is through their forum which I have gone to many times for help and always gotten timely responses and solutions. Lots of videos for help too.
In addition the plug in is stable and often updated with added features.
I highly recommend this membership plugin.