It doesn't give viruses.
The program itself does not give any sort of virus/adware/spyware of any sort...
It depends on what you download from it. You need to be smart when downloading from P2P clients. All, or most, connect you to the same network.
On the network, is music, movies, videos and whatnot.
You need to be careful of what you're downloading.
For instance - If I were to download a song. A typical song ranges from 2MB to 7MB. You can find out all this info before downloading. Just look at the description of the song you want, and it will tell you the size.
If you see a song you like, and it's only 500 KB, then you better not download it, because it's obviously not a song.
See what I'm saying?
As long as you follow a simple rule like this, you should be fine.
Yes, it's free. Use it sparingly, however.
I like iTunes a lot, it's so much easier, and you get what you pay for