I like a lot EZ Printswww.ezprints.com
For my color calibrated flow, they produce the best color match to my images when compared to other mass market online print services. The results are quite good indeed.
They have ICC profiles you can download for proofing. You can request test sample prints to get a sense of the match between your computer and the prints.
Quite fast service, usually 24 hrs + shipping.
The website is not that polished but it works well for me (I use the classic site, for professionals, on the left when you go to www.ezprints.com