This could rapidly become a thread and topic of its own, feel free to tag me if you want to discuss it further and we can start a thread to really go in-depth.
I used Ofoto because I heard of them first, but after a little digging, there's a large topic of color-matching profiles, other undocumented post-processing, and quality of print things that I've heard. A quick search on Google pulled up the rec.photo FAQ pages, and the digital FAQ included a long list of digital print shops.
I converted my images to Adobe 1998 color space and then uploaded, and got great results. I later found an unofficial Ofoto color space on jetcityorange.com, which I haven't tried quite yet. I don't see it visually as all that different than the Adobe profile, but I'm not exactly a trained color scientist.
I just put in an order with Ofoto for 60 4x6s and 5 5x7s, and I'm anxious to see how well they really turn out.