I've used couchsurfing a couple times, both with great success.
After a 4 month stint at a cement factory in ireland, I decided to blow what I made backpacking in europe for a couple months before I had to be back in the states for the holidays. I found free places to crash in Glasgow, Paris, and Barcelona, and probably could have in other cities if I was more diligent about it. Let me describe all 3
1) Glasgow - I previously knew this girl and she told me all about couchsurfing and her wonderful experiences as a host. So, after my visa expired this was the first place I crashed while I planned out the rest of my trip. Her landlord was a softcore lesbian porn producer from london (no joke here) and she basically lived in his second apartment while he lived in london. Apparently he only used the glasgow flat to store crap and occasionally shoot movies. I got to meet the guy, really rich creepy asian guy in his late twenties, but other than that it was fairly uneventful.
2) Paris - I used my couchsurfing profile with me newly rated as a guest by my host, and was able to find a girl slightly younger than me (her - 19 me - 21) who was willing to host me in paris. She pretty much lived with her boyfriend, so her old room was free for me to crash in, right in the 5th district of paris, around the corner from the Parthenon... so that was pretty dope. Plus she had friends around my age that I got to party with, and she worked at a pasta stand so my broke ass got free food.
3) Barcelona - This experience was slightly more weird. I found this guy, Warie, Jamaican guy, very last minute. He lived in Ceranyola de los Valles outside barcelona. Due to my shitty spanish and poor timing with the trains, I didn't make it to his place until much later than expected, and ended up sleeping in the stairway of his apt complex while I waited for him to wake up for work. After that I didn't see much of him as he spent most of the day working. His wife was always around to let me in, but she was very quiet, and I ended up leaving after a weekend to go back to paris for a while on my return trip.
Overall it was pretty awesome, and saved me at least ~400 euro on hostel costs. I'd say using couchsurfing is very different for a guy or girl, and probably much harder to travel in groups. My friend who told me about couchsurfing said that its very easy for girls to get places to stay (obviously) but they need to be a bit more cautious.
Anyway, that's my story, couchsurfing is the shit!