I just picked up four 12x18 digital enlargements from my neighborhood Costco, and I must say, they are excellent.
I used two images, shot at full-resolution (6.3Mp) with my 10D, cleaned up and fine tuned in PSCS2, then I saved the TIFF files and then made two duplicates of each.
The first of each shot I converted to 8bit, JPG, changed color space to sRBG, and saved at max resolution, size 12 JPG.
The second I did the same thing, but used view proof setup, changed it to the custon ICC profile supplied by DryCreek Photo for my local Costco's Noritsu printer, and saved it with the applied profile (no ICC profile embedded, the Noritsu can't read it).
Both sets of images were uploaded separately, as two orders, to my local Costco Photo dept., and "lustre" paper was selected, as was "no enhancement".
The plain old JPG files came out beautifully, and if I did not have the other two with the custom ICC profile applied to compare against, I would say they were perfect.
The ones using the costom profile for the Noritsu are fantastic. I don't think I'll ever waste paper, ink and time printing anything bigger than 8.5x11 on my i9100 again. I can't see any difference between the Costco "best" prints and the same image created on my Canon i9100 on Ilford Gallerie Classic Pearl (with appropriate custom ICC profile), which is my "benchmark" combination.
I would highly recommend following the workflow described on DryCreek Photo's site, and using Costco for hardcopy, at least up to 12x18.
Just my $.02