I've always used Costco for my 4x6 family shots and for our Photographers for Freedom project. They were always fast, good, and CHEAP. Last week I decided to try some 12x36 panoramas and to also develop a poster print that I could use as promotions. Our Costco here in Jacksonville uses a Noritsu 3411 digital printer which is a $200,000 beast that cranks out 2000 4x6's per hour. To get some decent prints you need to use the printer profiles on the Costco Photo Center web site. Go to the bottom of the page and click on Printer Profiles, and this should take you to some pages by Dry Creek Photo (the profile providers). On these pages you can locate the Costco store in your area and what profiles you will need. Follow the instructions to download the profiles and then do the softproofing in Photoshop. Before you upload you need to convert the image to the profile while saving your JPG. All of this is described in detail on the site, but if you have any questions let me know. When ordering online there is no option for the larger sizes, so I just upload as a 4x6 and then give the boys a call to tell them what's coming. Make sure you specify NO auto corrections. Steve and Allen at the Jacksonville store keep their Noritsu tuned up everyday, and I've never gotten a bad print. Oh yes, the cost of a 12x36 is $4.90.
Warning: one time I exported a set of 4x6's from Lightroom with a ProPhoto color space assigned. The colors on the prints were awful. I took my laptop down to the Costco store to determine that this was a problem. If you are not going to use the supplied profiles, be sure to use either Adobe RGB or sRGB color space when exporting images.
Our Costco is expecting a new printer in September that can produce 20x30's for $8. I can't wait!
For more information on preparing your files for printing at Costco please visit this excellent blog post by photographer Philippe Cailloux http://photsy.com/blog/?p=457.