For small prints, I generally use Walgreens and am happy with them. I found them to be equal to Costco and more convenient for me. Adormapix paper choices may be a better, but for normal prints I guess I don't really notice the difference - or care. For larger prints, or if I'm making a lot of prints to share, I generally use Adomapix and am happy. I guess I figure if I want a larger print or to share with family, which isn't too often, I'll pay a little more to make sure I'm getting top quality. But even then, I wasn't unhappy with Walgreens or Costco large prints. I used to try other places online, shutterfly, whatever. But I was unhappy at times and stopped using all online sites except Adoramapix. I think that for normal prints from local stores like Walgreens - it really depends on if they service the machine properly. I guess my store does a good job because they come out good.
For Calendars, I've used Costco and Walgreens, and found Costco to be quite a bit better. I've only printed a few books, but the Adomapix one is far superior to the other ones. The other ones were much less expensive but it shows. I made a nice mug at Walgreens which is much nicer than I expected - it came out great. I guess my finding is that it depends and there may not be a clear answer other than trying things out. But for books, which take a long time to make nicely, I intend to spend the money and use Adoramapix.