Printing can be a particularly fraught experience for artists.  We care more than most about print quality, color accuracy, paper texture and weight, reflectivity, and so on.  Admittedly I haven't tried an exhaustive range of companies providing this service, but I always appreciate others' reviews so I figured I'd detail my own experiences here.

Overnight Prints

I used to order from Overnight Prints for business and greeting cards; they had decent quality products at the low end of the price range when printed well, and though their color calibration/correction was often off (thus not printed well), they would reprint if asked when a batch was unacceptable.  Their online software was quite easy to use and their item creation instructions were very clear.

However, the last time I used them they wildly oversaturated the color on about 80% of my products, and this time when I requested a reprint they told me that the pieces were within the range of acceptable variation and refused to replace or refund the pieces.  I had to eat the loss, and it was a fairly sizable order for me (though I'm sure it was a drop in the bucket for them).  Since they have always been so hit or miss in terms of quality and have now changed their customer service policy and are unwilling to reprint poor quality runs, I will not be using or recommending their services again (I also made this politely but firmly clear to the customer service representative, who did not care).

My original rating for Overnight Prints: B- (moderate-to-low erratic quality, low cost, good software, good customer service)
My revised rating for Overnight Prints due to refusal to reprint poor runs: D (moderate-to-low erratic quality, low cost, good software, poor customer service)

AdoramaPix

While I was futilely arguing with Overnight Prints about that last order, I happened to need to make a photo book for an upcoming exhibition.  I'd never made one before, and after doing extensive online research I ended up going with AdoramaPix.  They had a reputation for providing a reasonable price point, high quality pieces, and fairly accommodating online software (which was particularly a relief in the face of a couple companies that required you to download complex proprietary software).  The process of creating a photo book with AdoramaPix was quite smooth, and the end result was even nicer than I had anticipated. It looked professionally printed and bound despite the fact that it was a single book run, and the price was lower than I expected for such a high quality item.   I will note as my one negative regarding the book creation process that their FAQ and online forums on photo books are quite out of date, though, and none of their contact points for their FAQ/forums/Facebook responded to me when I asked a couple questions.

Next, a student of mine wanted to print photos onto glass.  I told her the few reviews I could find online of the company she wanted to do the latter with, Fracture, weren't so hot.  She ordered a couple samples from them just to see, and it turned out that the panning of the technique online due to high noise in the end imagery was justified.  The Fracture prints were not acceptable for exhibition.  I recommended she try AdoramaPix to test out their aluminum prints, which I thought might be similar aesthetically to what she was going for but with improved quality.  She ordered a couple samples of the aluminum prints, and they were much better than the Fracture pieces.  She then ordered a large batch, and again the quality was high.

Then AdoramaPix decided to branch their business out into printing greeting cards.  I was in the market for a new printer for these since I'd decided not to do business with Overnight Prints anymore, so I gave them a go.  Their online software for greeting cards was atrocious.  It wasn't even functional in Chrome, and in Firefox it was still a huge mess - it randomly readjusted the size, typeface, and placement of text on the card; wouldn't display the software correctly in any browser such that I had to zoom out to about 50% scale to access necessary tools that I then had to try to use properly while tiny; and some other similarly frustrating issues.  I decided to risk trying the cards out anyway since my other experiences with the company had been so positive and left a long comment in the order asking for their review and oversight since the software was so glitched out.  I also asked them to review the paper choices I'd made as inexplicably they don't have images of all their papers viewable online (and in many cases the few viewable ones they do have must be independently found in videos on YouTube as opposed to through the building process on their own website).  Dealing with all this grief paid off in a big way, though, when I received my order: these greeting cards also exceeded my expectations and knocked even well-printed items from Overnight Prints out of the water in terms of their professional, high-end appearance.  The printers had clearly read my requests as not only did the cards turn out great despite the software, but they also adjusted the paper type of one of the cards to better suit the product.

I recently reordered some cards, and it appears that they're working on the greeting card software as it was less broken than before - I could use it in Chrome this time - but it does still have some fairly major issues in terms of image and text placement, image rotation, and text size.  I just keep leaving them notes in the order section asking for their review, since that seems to have functioned well so far.

I have now expanded into ordering aluminum and paper photo prints from them as well as the occasional framed print (the framed print software also suffers from weird problems; their online software/site issues are really their Achilles' heel).

I did have to request a redo on a framed print I ordered as there was a scratch/dark hair-like mark on the photo paper (not an actual hair though - it was part of the paper itself).  They requested photo documentation of the problem but then sent me a whole new framed print at no charge.  That means that so far, their customer service is great too, and when I got the new framed print I was happy with the quality.

My rating for AdoramaPix: A (great quality, moderate-to-low cost, poor software/online support, good customer service)

Moo

 I recently had to print off a new batch of business cards, and I needed to move on from Overnight Prints due to their lowered standards.  Honestly, I've been so impressed by AdoramaPix thus far that if they did business cards, too, I would have gone with them.  AdoramaPix currently doesn't offer business cards as an option, however, so I went with Moo.  They have a reputation as being one of the best companies for artists, and I liked that they let me use multiple backs with a consistent front.  Their cards are quite pricey - they cost a little over 35 cents per card at the current size of order I'm making.  For comparison, Overnight Prints charged me around 10 cents per card.  I may end up trying out more printers to see if I can find a more affordable printer, but in the meantime, the cards from Moo are beautiful and I've been asked several times about them by people who have admired their quality (and design, but that's my own!). 

My rating for Moo: A- (great quality, high cost, good software, customer service thus far unnecessary)