I have the xvest. I use it when doing bodyweight exercises (pushups, pullups, dips, box jumps), and sometimes when I'm just walking around my house. It's just another tool for me to keep things interesting.
Here is an article regarding running with a weight vest. It sounds promising; however, the weight must be slowly added to minimize risk. I've also heard of people running up stairs and hills with vests to minimize the impact on the knees.
The xvest has a nice tight fit, and the weights can be evenly distributed between the front and the back in 1lb increments. My only complaint is that the shoulder straps are a bit clunky, and get in the way a little bit, but not enough to cause any real problems.
There are quite a few brands out there, but here are some of the more popular vests: