Whether or not it is good depends on what you're doing. Sweating is your body's way of regulating your body temperature. When you are working out, your body temperature goes up. To combat that, your body starts to sweat to try to lower your body temp. The more hydrated you are, the less you will sweat. If you are working at a real high intensity level, chances are you will sweat a lot, which is normal. If you are doing mild exercise and you are sweating a lot, chances are you are dehydrated. As long as you drink plenty of H2O, you needn't worry about the sweating. And make sure you carry around a hand towel at the gym. You're going to sweat, and most people (myself included) get annoyed when people leave puddles of sweat on benches and machines.
As far as what toxins get released when you sweat, I don't have an answer for that (other than sodium).