One of the challenges that some ecommerce merchants have, particular those who sell digital goods, is recurring subscriptions. If you have a very simple $X/month model with a small number of people then the PayPal recurring subscriptions is probably good enough.
Early Impact, makers of ProductCart, just release SubscriptionBridge, a service designed to provide a more enterprise friendly subscription service. The differences between this and PayPal are numerous. One key one is the ability to not use PayPal! It supports Authorize.net as the payment gateway. It has hooks into Magento and of course ProductCart with more to come. Worth checking out if you need to handl recurring payments in a scaleable way.