A strong vote for MPix. I've never had a problem with them. Sadly, I can't say the same for Adorama.
About a year or so ago, they were running a special and I had a lot of 12 x18 prints to make, so I sent a batch to Adorama that I had previously had printed at MPix. Thought I would save some money. Big mistake.
They weren't terrible, but the colors were much more dull and and just not quite right.
With MPix I never have to do anything to the files other than upload them and pick the sizes and crops. Even if I am sending something where I've done some extensive effects (With some subjects I like very muted colors and will blend grayscale and color layers to get the effect I want. Other times I may need very vibrant almost garish colors. I'm also a heavy user of OnOne and Nik.)
It always seems that MPix knows exactly what I'm looking for and prints it perfectly. (Without any special instruction). In fact, even when I've sent the same file back to them because I needed more prints or needed different sizes, the prints come back almost perfectly matched to what I have ordered in the past.
I agree regarding MPIX. My only complaint is that I have a real hard time matching brightness from my monitor to a print. I use a Spyder 3 to calibrate colors, and I have brightness in my monitor turned down to 20%. Invariably I need to print, up the brightness/contrast, print, and possibly up the brightness again before I'm ready to print an enlargement. It's not an MPIX problem, but my frustration with the whole digital printing process.
I've recently tried Nations and think they're very comparable to MPIX....both get my recommendation.