What Spreedly is offering (transparent post to payment processor, redirect back to retailer) is the same as what Braintree is already offering with PCI compliance built in.

That said, we're not happy with Braintree since they messed up an account transfer of ours.

The biggest point I'd like to make is that it sucks once you've got your customers in a recurring plan (Spreedly, Braintree, PayPal, etc). If you're not storing their card numbers, the attrition rate should you choose to change processors is unknown and possibly greatly damaging.