Body armor works fairly well at preventing abrasions and cuts, but does not do as well against impact damage. I wore a SixSixOne pressure suit for many years when downhilling. Basically during all the slow and medium speed crashes it keeps you from walking away bleeding from rocks, bark and maybe small punctures (from exposed tree branches and such) depending on where they land. It doesn't keep you from getting bruised (that much), and will do little to mitigate the forces you encounter during a high speed crash. I still managed to dislocate my collar bone during a high speed crash in full armor. However, I wasn't bleeding where my shoulder slammed into the tree, so there is that. I'd sum up by saying body armor is NOT like a helmet for your body. It will not protect your spine from a high impact crash, but it may keep the skin on your back from getting torn up.
As far as what to look for, I recommend the same things you's look for in any cycling gear. It should fit well and be well made. Depending on your riding conditions and weather, ventilation can be very important. Even at 75F, my armor made me sweat like the dickens. At 85F or 90F, I don't think I could have stood the heat, but I was used to riding in sub arctic (interior Alaska) conditions.