Printcarrier are by far the cheapest and the quality is excellent. Although you need your own design all they do is print. We get about 2000-2500 out a week but that is by its self and they definitely go it ( as its us doing it)
But unless you do what we do I would never suggest doing leaflets, the delivery is the most important bit, the guy on the window cleaning section who has just got 80,000 leaflets and is asking who can deliver them is setting himself up for an expensive lesson.