I use WHCC and definitely recommend them + MPix.
I've used White House Custom Color too (their lab is only about 90 minutes from here), for prints where I want to tightly manage color (they want an ICC) and some album creation. They are excellent, but very much oriented toward professional photographers. I started using them many years ago (they were called something else then), mainly for making slides from PowerPoint, back in the day before digital projectors. They had the only upload service anywhere (an Apache BBS using FTP) and they were (and still are) utterly reliable with excellent customer service and their proximity got me a 24 hour turnaround.
I've used Kodak for years too (it used to be Ofoto) because the ordering process and account management is simpler IMHO. They have a very intuitive web interface (you don't have to use their proprietary software) and they ship promptly. I have never been dissatisfied with prints from them, from 4x6 up to 20x30. The other Kodak feature I've used is the ability to share your online albums with people you designate and let them order their own prints. That's nice for snaps at family reunions, local little league games etc...those situations where you take a bunch of snaps at some event and people ask you for copies. I'm not a professional photographer and that situation is common for me. WHCC used to accept files via FTP only, but they now also have some sort of upload interface that is supposedly simpler and allows viewing the file and cropping for different print sizes like Kodak has. I haven't used it. And I always do my own cropping anyway.
Bottom line, if you're a professional photographer and making prints for a client, want to tightly manage your color, calibrate your monitor, send embedded ICC info, and make sure the prints are identical to your monitor output, WHCC is first class. KodakGallery is simpler, a little cheaper, and very high quality IMHO.
