The colors are more saturated and the paper it's printed on is better. Shutterfly and Snapfish are good too, but for some reason Ofoto has a marked leg up on quality. The pictures look VERY professional in terms of its quality. What's also great is that they are consistently fast in delivery. I get my pics back in the mail in a couple of days. Every other developer has taken 2-3 weeks to get. The slowest being Dotphoto.
I'm not the only one who has commented this. People that look at my photos from Ofoto comment how amazing they look and ask what camera I use. The same pics with other developers look flat in comparison--especially the cheapest online developer of the bunch: Dotphoto.
I don't print much as I perfer to watch my photos on the screen. The ones I print are special pics and I want QUALITY. In my extensive experience, Ofoto, at this point in time truly has no equal. If you look at magazine reviews, they are all in agreement with Ofoto as well in terms of picture quality. I'm not the only one who thinks this way.
Costco is on the level of the rest and has good prices. Pics look OK. Pics don't look as great as the Ofoto guys though.
Disclaimer: I don't work for any of these companies. Just a guy who takes pics as a serious hobby and who's worked with all these online photo companies.