Most of them have some sort of virus or malware, but if you don't click on any adds/popups/etc, you can usually avoid getting anything bad. Also, I wouldn't recommend using them on your computer, that is just a bad idea. If you have an iPad or something similar, that would be best (just because often times you have less stuff that can be stolen or corrupted on a tablet or iPod than you do on your computer).
This. Also, depending on your Internet Service Provider, quite a bit of them monitor movie streaming and downloads in accordance to piracy laws and can send you a letter to stop.