Although I now print mostly on my own Epson 3880 and R1900, I still send larger prints to MPIX, and used them for many prints before I got better printers. After testing several labs, they seemed to produce the best rendition of that I thought I sent to be printed.
I have no qualms about printing a 16x20 or 16x24 from a properly processed image from a 12MP camera. I've even had a 30x60 inch print made from a D300 image. It's all in the processing that you do to the file.