I've had really good luck with Nations Professional Photo Printing & Photo Gifts | Nations Photo Lab prices seem comparable, quality is excellent, service is quick, and customer service is really great! I had a couple of 16x20's mounted that the corners got dinged a little in transit. When I called them, they just said "we will ship out replacements today, via express you'll have them tomorrow". Can't beat that.
Another recommendation for Mpix - great quality prints, and very friendly customer service when needed. My only complaint is the price - adding a lustre coating triples the cost of smaller prints, and the shipping costs are pretty high for smaller orders.
I was pretty happy with AdoramaPix as well, I just thought Mpix was slightly better when compared side by side. I would definitely use them again though.
Costco was actually pretty decent once I had the correct printer profiles downloaded and applied (Digital photo lab profiles). However preparing photos to print through Costco can be quite labor intensive, and is a step i can get away with skipping with other printing services, such as Mpix.
Tony Northrup did a decent comparison of popular printing services a while back as well.
I use AdoramaPix and I've been pretty happy with them.
I had them make a large canvas once about $200 and they sent me a message that they liked my picture and enjoyed working on it. Thought that was interesting.