I tried both guesting and hosting many years ago. It wasn't via couchsurfing.com but via the net nevertheless. I have heard many people report great experiences. But in my case, both kind of put me off.
When I guested, the host took more than the usual interest in me and directly asked for sex when I wasn't taking the hint. I felt really uncomfortable after that (and no I did not give in lol). Come morning, I found a hotel room to spend the one more night I had in that city.
The time I hosted, my guest turned out to be a bit of a slob. The entire 5 days he was there, the bed never got made, there was dirty clothes all over the floor and though he used all my dishes he never washed them after. Mind you, my apartment was really tiny (so it was hard not to notice) and I did not have a dishwasher either (back then I was really extreme and had just a pair of each dish :)). Everyday after taking in the city, he'd come back with exactly one pastry -for him to have next morning (on top of the free breakfast I was offering him). I thought that was a hoot (and no - considering all the shopping he did - he wasn't simply being frugal).
Maybe I just ain't flexible and open enough.. but I thought it was all a bit much and never did try guesting or hosting strangers again. Instead I now only do it with friends I already know or people I have met online and got to 'know' rather well over many months, preferably years. I live in a very expensive city, so there's enough traffic as it is. :)
Mine was just one experience. I keep hearing lots of people online say how wonderful their experiences have been. I'd say, try to get to know the person a bit and give it a spin if you feel comfortable!