I've never had problems with Priceline, but do prefer to remove middlemen if possible. Can you find the same fare right on Delta? If not directly, maybe via a Kayak link? You might have to use the multi-city routing to pick up the segments you want.

I'm not sure you necessarily get mileage with Priceline. If you are interested in getting it, you need to verify that before you purchase from PL.

Romelle