Same experience as couple of the up-thread posts.
Smaller than 8"x10" - shutterfly.
Large than 8"x10", go with mpix.
Tried the large format (16 x 20 & 20 x 30) at Costco due to their low prices. Results were very disappointing. Somehow, they must have changed the "temperature" to a much "warmer" tone for the larger prints. I have identical images printed on the same day, the 4x6 came out great; but the larger format prints suck. They refunded me.