I'm on my second unit made by EMPI, and I swear by it! It's the only brand recommended by my Pain Management Center. I've used it for back pain and it works like a charm. My unit was prescribed by my Pain Management doctor, so it's cost is covered by my insurance, which is a big plus. My unit comes with a variety of electrodes and settings, and can be used on various parts of the body. I will tell you this...on the days that I'm totally out of my pain meds, which happens every week, my TENS unit is the only thing that gets me thru the day. There have been times that I probably would have considered checking out if I didn't have my TENS unit to provide me with pain relief...I value it that much!
Most often I use a butterfly electrode on my lower back and turn the intermittant setting up to full blast. The "zapping" takes a little getting used to, but I wouldn't trade mine for the world. The only reason this is my second unit is because I accidentally lost the first one on a trip, otherwise I'd still be using it. Ebay is a great place to get cheap replacement electrode pads, which are considered a "consumable" item and pretty much interchangeable. They usually only last 15-20 uses before they are shot. My EMPI unit came with two sets of rechargeable batteries, and each set lasts several hours on a charge. This is a feature you definitely want, or you'll be going thru a boatload of batteries.
Well, that's my recommendation! Let us know what you do!