Hi
My wife Deb has a pair of MBT's and loves them. They are very different from normal shoes though. I tried a pair on and walked around the shop, you have to think far more about your balance and walking properly at first, although they are a pleasure to walk in.
Deb has a muscular imbalance in her thighs, a long term problem that causes knee pain during certain movements. The MBT's should help correct this, but she has not had hers long enough to find out yet.
Personally I would have a pair if I walked on flat ground enough, but as most of my walking is on rough ground with our dogs I did not feel they were suitable for me. Very nice and comfortable to walk in though though...
They should help to some degree with majority of general back pain because they encourage you to walk with good posture, and by holding your body correctly most people will increase the muscular endurance of their core muscles. Core muscle endurance and strength is important as it is these muscles that should be helping to protect the spine during our daily movements. It is difficult to walk 'badly' in them. Majority of back problems respond favourably to an increase in core muscle strength and endurance.