1. The question of safety is too general to answer. Each brand has a different proprietary blend... some are basically just caffeine, others NO, others B12, ginseng, etc.
Most of them are marketed as dietary supplements, so they fall under the Dietary Supplements Health and Education Act, which basically shifts the burden onto the FDA to prove that the product is unsafe.
There were recent deaths, heart attacks and other severe side effects associated with the use of Jack3d (USP's pw). USP recently stopped distributing its original formula due to the FDA's request, however, most of those stories involved either abuse of the product (grossly more than recommended dosage), an underlying heart condition, or extreme physical exertion in extreme heat following use . So you have to take those claims with a grain of salt.
2. I have personally used a few types in the past (jack3d, white flood, craze, fierce... and a few others), and I responded differently to each one. I loved two of the ones listed, one made me feel sick, and the other one had no impact. However, everyone responds differently. My friend swears by the one that made me feel sick. The two that I liked definitely gave me a lot more power, energy, focus, and endurance.
I stopped taking them for now. Although I found them beneficial, I would suggest saving your money if you are still getting good gains. I think getting the proper sleep and nutrition is more valuable.