I have been a couchsurfer and a member of couchsurfing.com.
I stopped participating after i got married, so i can concentrate on matrimonial bliss
but yes it is safe, it depends on the website you use, i would only recommend couchsurfing.com
I have made allot of good friends and had excellent experiences from Australia,Netherlands, Belguim,UAE, Thailand,Malaysia,Philipines and Tokyo
I have hosted people from Portugal, Australia, Canada, and Turkey.
I have met allot of weird and wonderful people including some crazy people, and i had the time of my life.
Sometimes i would not plan crash at someones couch but have my own hotel room, but after meeting up with the local couch surfing community at their local hangouts or events i would change plans and end up on someones couch in some other random city that i did not plan for.
This happened to me in Amsterdam a couple of years ago, i was there for 3 days and then after checking the calender there i discovered their was a get together down the street i was staying in. Met some crazy characters, partied hard and even narrowly escaped getting raped in the toilet by a fat turkish women that was very into me.
Long story short, a group of couchsurfers and local amsterdamers were heading to Bruges in Belguim the next day, so i cancelled my hotel room and joined them for an interesting time in Bruges.