I have two and LOVE them! My daughter uses the half pad and I have a larger western one to go under all of my different saddles. They really do seem to work to keep the horse comfortable and moving more freely.
If your horse has back pain, I would get that one along with the front and rear shims. The Ultra ThinLine material on its own is good for prevention of mild back soreness/fatigue, but the thicker 1/4" ThinLine material is better for sore backs or saddle fit issues. If you have the shims, you'll have double protection. You can always take the shims out and only use them for hard rides or when his back comes up a little tender.
ThinLine has a 30-day guarantee, so if you don't like it you can return the pad for a refund.
Also, if he has back issues, you might want to contact a equine chiropractor (I prefer one that is also a vet) to have his back checked and his saddle fit/placement checked. A good massage therapist to keep his back going between chiro visits might be a good idea too.
Oh, duh, I just read that it's YOU that has backpain! lol I read your post too quickly and thought it was your horse. Anyhow, yes, the Ultra ThinLine does well for riders. My daughter says she definitely notices a difference when she works on sitting trot. She says it's always a bit rougher without her pad.
Other people I have talked to say that it helps over time. You don't notice a big change right away, but after a few months of riding with it, you'll notice that your back isn't as sore as usual and less fatigued.
You might also want to buy the book "The Rider's Pain Free Back." It was written by a doctor who specilizes in back pain. It makes mention of ThinLine pads as a benifit to riders experiencing back soreness.