I couchsurfed through europe in the mid to late 2000s and had a great time. At the same time I was hosting a fairly popular flat in Glasgow. Half of our guests came through couchsurfing, the other half were word of mouth. Best experience was two italian climbers (who we met bouldering, not via cs) who ended up with tents literally set up in our lounge room on and off for a number of months. These guys were proper characters, just young guys who'd lost their jobs as bakers back home and had decided to "climb" scotland (in between stints at ours they literally walked and climbed their way up and through the very northern highlands). They had very little english and existed on a diet of stale donuts and nutella. Last I met them was on a trip to Venice (I was back in Australia by that point), one of them travelled down to meet my wife and I from his latest adventure setting a remote cave in the north of the country. By that point his english had improved immensely and we finally were able to have the conversations that were never quite possible in Glasgow.
Counter that with my worst experience. We accepted a request on CS from a young brazilian artist to stay for a weekend, over from her studies in Italy. I tended to make a point of accepting people with limited reputation ... I had some things I looked for, but generally if someone looked like they might not get first pick elsewhere we would accept them (non-english speakers with terrible profiles were the norm). In this case we were a little busy but thought she would be an easy, low-effort fit for the household. Especially since one of our roommates was also a (lovely and extremely outgoing) young brazilian artist. Our visitor arrived and seemed cool and relaxed; just in town to see the sights. So my wife and I did the usual and took her for a walk through some of the nicer parks and buzzier areas near the flat. One strange thing happened at this point (that I didn't pick up on at all) ... she was smoking, and making a point of talking about being a smoker. I don't smoke, but my wife later commented that it was obvious she wasn't a smoker. I thought it was odd but didn't question it. Anyway, long story short, by the end of the weekend the "visitor" had revealed herself as a stalker who had found our brazilian roommate through her public facebook profile and had begun an "art" project on her, which involved collecting her public photos and stories for a diary / portfolio, imitating her in her every day life, and finally tracking down her CS apartment through some very basic online sleuthing (our roommate wasn't mentioned in the account) and joining us for a couple of days. She rounded a couple of us up on her last day and admitted everything, including the bit about not being a smoker which hit me for six (because it had already been brought up!). She then went to our roommates work (a local bar) and laid everything out to her while she was working .. which led to our roommate being fired for literally losing it in front of everyone. She wanted to go the police but in the end we just reported the couchsurfer. A totally wild experience that was the beginning of the end of our glasgow hosting adventures.