Paypal has an unfortunate history of treating merchants poorly. For example:

http://blog.apparentsoft.com/business/124/is-paypal-good-for-your-microisv-business-a-short-paypal-horror-story/

It can be really disastrous if your business is built on top of their platform. For example, if you ever need to move your subscribers to another system you could lose a very large percentage of them.

EDIT: Another one here. Beware.
http://cooking.fourbeansoup.com/post/322154074/paypal-is-evil