We have migrated 3 of 103 site we own running s2 to Simple Membership.
It does just about everything we needed that s2 does AND uses the ‘new’ API. Not the (mostly) deprecated IPN/SOAP s2 uses.
The Paypal buttons work fine on Simple Membership.
We have had issues on ALL our sites with the s2 PayPal buttons, with registration (because the return POST variables are empty).etc.
Simple do use a separate database for users/members, (s2 uses the WordPress tables and just add a bunch of meta stuff)
but Simple has a plugin to transfer WP users to Simple.
They have an API for custom stuff.
We are going to continue testing Simple and if s2 is not fixed soon, start migrating all our s2 to Simple. May even create a PDF on how to do it.
Over a month to get a fix is just not reasonable.
My CTO also informed me that out on the net the word is PayPal will soon REQUIRE encrypted buttons - encryption and the using the old API are the major problem. If they do that, a LOT of sites are going to crash
Sid