Airbnb is awesome! Used it while I was travelling all over Europe interrailing. I'd recommend using it when you get bored of hostels as it gets a bit pricey after a while.
Make sure you book well in advance, so that you don't get absolutely mugged off by some of the steep pricing. Some hosts idea of a "fair deal" is an absolute joke.
Travelled alone and stayed in AirBnb's without issues. As the above users have stated, go with rooms/flats with reputable reviews, with good pictures of course.