1. The programme itself is safe. But it connects you to totally unvetted fellow users and is not capable by it's very design to control what you download from them. It is important that you have adequate virus protection when using any internet application. There are viruses out there and they are not uncommon particularly but not exclusively in programme files, and quite honestly you are best advised to get them from the proper sources. If you go to search ... above right and make a search for virus you will find posts on the subject.
2 you can share/not share exactly what you wish to.
3. After it has been running for several hours it is probable that your computer will slow down. You need a good 2GB of free space on the disk which holds the application and your OS otherwise it is possible the computer will slow to a halt.
4. Where to start. I couldn't live without it
You will probably find it useful too look through the sticky messages at the top of the forum list. These will give you an idea of what to expect.
You say "switching to" .... from what?
I hope this helps