Disadvantage - you have limited number of concurrent connections and and anyone with access to the network can access all shared files/folders. You open yourself up to viruses/spyware/adware/rootkits and lots of other 'malware' Its generally used to by peadophiles to distribute child porn (on the assumption its harder to trace, than just putting it on a website) There is lots of music available which is in breach of copyright, which adds to p2p's already bad image It eats up a lot of bandwidth, which you will only fully understand when you get your ISP bill at the end of the month
Advantage no extra cost in setting up - share peripherals Its a good way for small bands / artists / video makers etc.. To get exposure Its a good way for people to share files on the internet
