I just had some prints done at MPIX...Decent size, they were both done in 12x18. One was a portrait of my son in BW and the other I have posted on here before. For the portrait I had it printed on the BW paper and for the other on the Metallic paper. All I can say is WOW...Both prints turned out fantastic. The Metallic paper is really quite impressive.
The shipping was cheap and very fast. I would recommend MPIX to anyone who needs prints.
I am going to try out WHCC next. They offer a few different options and if I remember correctly they have free shipping and can accept AdobeRGB files, for those who use it. I will post up my experience with them at a later date.