When we updated to WP 3.1 and s2 1.51, all the custom registration fields disappeared. They are still in the database but gone from s2 General Options. Additionally, the membership descriptions. login welcome page, and many others get changed back to the default. Those we can live with, but losing the custom fields makes it impossible to meet our clients needs.
What we really need is the ability to integrate with Gravity forms. Those forms work.