We've just had 50000 flyers done with printcarrier.
This was a risky step in some ways as any real editorial control is lost. The cost was the main incentive, especially in the current climate. At just over £500 for double sided print this worked out as almost half price on our regular, local printer (for which I do feel a bit guilty)..
Whilst there were a few scary moments along the way (we opted to approve the proof prior to print and this didn't happen, despite paying for it), the flyers arrived in good time and looked the part.
On this basis I would therefore recommend printcarrier. Definitely opt to authorise the proof and speak to them along the way and I'm sure you'll be happy.