I'll take a shot at this since I used to work in the industry for 10 years.
After hours trading is very risky. Most of the time, large swings you see are made because of some kind of information being released. If you do not know what the information is, you can get burned very easily. For example, a company might release blowout earnings sending the stock up in after hours. However, if you didn't listen to the earning conference call and they mention a slowdown in orders, a product line not doing well, a huge charge, etc. stock could completely whipsaw. This happens all the time. You really need to be on the ball if you decide to do it.
At my old hedge fund we used to buy aftermarket on Instinet quite often but I would say we broke even.
If you are trying to buy/sell stock after hours where nothing is happening, chances are you will not get a good price because the volume tends to be very low.
So unless you really know what you are doing, I would stay away. A lot of the moves are really psychology driven and that could change in an instant.