Never marinated in it but I have used it in the foil on some baby back ribs this past monday for a little moisture during the foil wrapped portion of the cook. I was going for a slightly sweet end product and also sprinkled the ribs with brown sugar before foiling them. Can't say I could taste the Dr Pepper in the finished product but they had an awesome slightly sweet bbq taste.
I've also used it in the Dutch Oven with a brisket for a little moisture in the early stages of slow baking them. Again, couldn't taste Dr Pepper but it has a slightly sweet taste.