I have used the following:
My Publisher: The software was easy to use and intuitive, although they didn't allow for as much customization as I like. Good quality paper and binding, but they auto correct your photos by warming the tones up. If you're like me, and you correct all your own pictures, this can lead to overly strong reds and oranges. Also, I found their printing to be slightly pixelated, which I know is them and not me because of the quality all my digital images are shot at.
Snapfish: Has a great range of sizes and styles of book, more than others. I found the paper a tad on the thin side and the layouts more limiting than with My Publisher. The printing was decent, but not exceptional and I did notice some auto correction of my images where the colors were off quite a bit.
Blurb: My favorite printing site. The books are beautiful quality with very nice printing. Because they work with a lot of professional photographers they don't auto correct anything, which I really appreciate. The latest version of the software allows for almost complete customization which I also really enjoy. I wish they had more design elements included, but this is a small pet peeve.
There a few others out there as well, but these are the ones I have experience with. I always recommend Blurb.