Be sure, if you're considering a fee-based POD, to order one of its books so you can assess physical quality. Books from fee-based POD companies won't be reviewed in major publishing industry venues. Good reviews in journals like Publishers Weekly, Booklist, and Library Journal can be a major boost to sales. But most of these venues will only review bound galleys in advance of publication. Fee-based POD companies don't issue these.
Delays are possible.
No one seems to know quite why, but fee-based PODs are experiencing a logjam in book production and order fulfillment. There may be extra expenses. Some services that advertise themselves as free charge so much for extras that they're actually more expensive than services that only charge a setup fee. Others charge a high fee for things you can purchase on your own for much less, such as copyright registration. Royalty income may be less than you think.