Most folks who recommend mpix or pro labs do so because those labs frequently calibrate their equipment, provide consistent results, and they don't fiddle with your image unless you ask (and pay) them to do so. Professionals and serious enthusiasts take the time to calibrate and/or profile their computer monitors, then post-process their images in Photoshop or ther software to get them looking exactly the way they want them to look. All of that effort on the part of both the photographer and the lab are critical to getting a print that closely reproduces what the photographer sees on his/her computer screen. If any part of that color management isn't happening, then the results you get from the lab won't be predictable or consistent.
Most of your local labs in drug/grocery/discount stores use equipment that is mostly automated, technicians who aren't color specialists, and they don't calibrate frequently enough. But most heinous of all is that their systems automatically adjust the images you give them. You (and they) never know what the machine is going to do to the images as far as adjusting colors, contrast, exposure, saturation sharpening, etc..
So, to recap, using a lab like Mpix and having them not fiddle with the image will yield great results if the image you give them is already great (crap in, crap out). If you're not processing your images on a calibrated monitor, then the local 1-hour lab (this goes for the online consumer labs like shutterfly snapfish, winkflash, etc., too) may improve or worsen your image with their "auto adjustments". You wont' know until you get the prints. You could have mpix and the pro labs to have an actual person review the images and make adjustments to improve them. The results would be better than the auto-adjustments the consumer labs do, but you'll have to pay extra for this service.
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