I have the Waterpik brand. I have the big one in the downstairs bath where I have plenty of counter space and a mini rechargable one upstairs. The big one runs around $40. It is WELL worth the cost! I will definately get to those back teeth. I think Black & Decker makes one as well. They all work on the same principle. The water jet forces stuff out from between your teeth and under the gum line. It also massages the gums.
I think it is THE most important dental tool there is, IMHO.