This webpage also has some great information:
http://www.consumersearch.com/www/internet/digital-photo-printing/
The big take-away is that, in this very competitive market, the price typically indicates how good the quality is.
So Kodak EasyShare Gallery (Ofoto) and Shutterfly.com, while charging a lot more than the cheap photo finishers, produce far greater quality photos. Snapfish and
WalMart are about half the price, but provide much lower quality.
If you're printing photos for long-term safekeeping, then don't use Snapfish or WalMart. Rather, use them if you're just making prints that you'll have no concern about discarding after a few years. If you have your electronic versions of the photos backed-up in several safe locations (not all of which are in your house!), then who cares if you have to reprint them every eight years or so? If you'd like to save yourself the hassle (and expense, because
reprinting, of course, costs money), then use one of the two premier photo finishers, Kodak or Shutterfly.com.