According to the usage stats on http://drupal.org the module uc_recurring is now installed on over 1,000 sites (as of the 28th September 2010).

1,125 to be exact!

At Univate we have been involved in a number of membership/subscription projects. To achieve we use Ubercart Recurring Payments and Subscriptions (uc_recurring), which is a module that adds recurring payments support to the ubercart ecommerce modules for Drupal. Univate has been involved in actively development of the latest 2.x branch of this module.