What I like about Costo is that they're local (for me), fast (prints ready to pickup usually within 2hrs after uploading/ordering), high-quality, and have astounding prices ($0.13 for 4x6's to $3.00 for a 12x18). I just have to be careful how I prepare/crop my pictures before sending them. It took me a couple tries to get a feel for how closely they cut/trim their prints. They also do an excellent job processing film, for when you're feeling nostalgic or analog and they put high-quality scans on archival-quality CDs (really saves time).

I learned about mpix from Kelby Training. I was very pleased with their quality, fast shipping, and vast available print sizes. Their prices are higher than Costco's ($0.29 for 4x6's to $11.50 for a 12x18). But they deliver excellent results. The packaging is very good and secure. What sets mpix apart is the product selection they offer. Books, keychains, standout frames, wraps, calendars...they've got tons of ways to show your work.

One thing that both Costco and mpix do is print the filename on the back of each print. This is really helpful when you need to quickly find the photo again later. I highly recommend both of these companies. I have not had any problems so far and the service is excellent! Give it a shot and see for yourself.