Use utorrent,
1). No hardly ever, its safer than limewire and such and than downloading from random sites, unless your downloading software as some keygens come with virus's, Just scan everything you download and youll be fine
2). slow down pc? no, slow down internet ? yes, if your downloading using bittorrent, it will slow down your internet connection depending on your download speed, and some programs like bittorrent and azuires can slow down your pc as they're java programmed, i suggest utorrent as its programmed in C+ so its lightweight and doesnt slow down system preformance at all really.
Umm my opion i wont state as it's illegal

but yeah if you use it well it can be your best friend, i also suggest using www.thepiratebay.org for torrents as you can check comments on torrents to know if they're bad or have virus's and such and download a program called peer guardian as it blocks known p2p companys ip's so your safer downloading, and have a much less chance at getting caught

and Good luck!