MPIX is certainly a great lab, but it may not work for you, and here's why.
First of all, MPIX is owned & operated by Millers, one of the best labs in the US that serves only the professional photographer community. MPIX gives the same great quality results as its parent company, Millers, but it's open to the general public and has a slightly different catalog of products.
Back in the days of film, photographers were heavily dependent on their labs to fine-tune their images by correcting under/over exposure, color casts, by dodging/burning (lightening/darkening) selective parts of the image, etc.. With digital, photographer have taken over that part of the process by using the "digital darkroom" -the computer & software, like Photoshop. So, now that the photographers put so much effort into tweaking their images themselves so that they look a certain way, and making sure that their computer monitors are calibrated to a standard, what they need most from their labs is to print their images AS IS. They want a lab that will print the digital files to a standard similar to what they calibrated their own screens. MPIX does that well, so MPIX is popular with people who take their photography seriously. MPIX and other pro labs calibrate their machines and printers continuously to make sure that they're standardized.
When you look at your images on the camera's LCD and the computer monitor, you'll get a general idea of what the image looks like, but unless your monitor is calibrated, you don't really have an accurate portrayal of what the digital file actually looks like. So, when you get your images back from MPIX, they will be pretty accurate, but they might not match what you saw on your monitor. You can pay MPIX to "correct" your images for you (the way pro labs used to do for film photographers), but you will probably have to pay more for this, unless -and I'm not sure if they offer this-- MPIX allows you to turn on "auto correct".
Most of the consumer labs, like the one-hour labs, grocery store labs, discount store labs, and drug store labs, automatically apply some form of "auto correction" to all the images. Most of the techs who work at those labs can't or don't know how to turn "auto correction" off, which is one reason many of us don't use them for important stuff. Auto correction is not perfect. In fact, most times I hate what it does to images, but some times it can do a halfway decent job. It can do anything from global color correction, contrast enhancement, saturation boost, auto sharpening, global exposure correction, etc.. However, even if the local lab's computer, does a good job with auto-correcting the digital file, it could still wind up giving a bad print if the printer itself isn't calibrated. That's the real weakness of the drugstore-type labs. They don't change the chemicals or calibrate their machines nearly as frequently as they should, so what you get from them is totally a crap-shoot. Now, it's not usually so bad that a person wearing a blue shirt will look like he's wearing a yellow shirt in a print. For 90% of the people who are just printing snapshots, they're satisfied with the result. However, if you want really good results, you won't get consistent results from the drugstore labs. Slight variations can mean the difference between a person looking normal, lifeless, or sunburned. But, again, most people's eyes aren't trained to spot or care about those variations.
One exception to the local "drugtore-type" lab is Costco (or is it Cosco?..I shop at one and I own a ladder from the other). The labs at many Costco (sp?) warehouse clubs actually calibrated their printers fairly frequently and you can request that they turn off the "auto correct" feature. For this reason, I know some professionals who will use Costco (sp?) for proof prints when they're in a hurry (but they don't print on thick, professional papers with anti-copyright language on the back like the pro labs do, and they don't offer the range of papers and finishes that pro labs offer). Now, half the time their techs forget to turn it off, so be prepared for that. If they do turn off auto-correct (or whatever they call it), then you can expect prints that accurately represent the digital image, like MPIX. Remember, if your own monitor is not calibrated, that means that you could be in for a surprise when you get the print. Most digital images could use some form of correction. If you let them use auto-correct, then you'll be dependent on their computer doing a decent job analyzing your images and deciding how it could be improved.
Sorry for the novel, but I just didn't want you to expect miracles from the "as-is" prints you'd get from MPIX (or possibly Costco). They're pretty good at making accurate prints of the digital files, but as the saying goes, "crap in, crap out".
but a pretty good chunk of them. 
