About 3 months ago I didt a little test. I took a dozen pictures of various scenes (portraits, candids, foliage, costumed parade characters, etc.) and sent the same shots to 9 different places for processing.
I included all of the online processors I could think of as well as some "upload to our site and pickup at our store in 1 hour" national chains.
Looking at the series of pictures individually, most of the prints looked OK.
Once they were laid on a table side by side there were some really evident shortcomings in many of them - shifted colors, blown highlights, detail lost in shadow, etc.
I had several people - some with photography experience, some without - look at the series and pick their favorite bunch. I never told them my preference.
The decision was unanimous. Winkflash had the best prints. They were picked best on every print of the seriies of 12. They were followed closely by Snapfish. Cosco was up there in quality as well.
Walgreen's and CVS were at the bottom along with Ofoto.
Take this for what it's worth. Not very scientific, completely subjective, and a small sample size.
Your mileage may vary, but for now I'm sticking with Winkflash.