I've used it to rent my place out several times this year. It's a great source of supplementary income, especially if you're in a high demand city like NYC. One guest was a little pushy, and another guest seemed a little clueless and ended up losing the keys in the mail (at least I'm knocking on wood that that's what actually happened... insufficient postage maybe?), but for the most part things have turned out great.
I have not used it as a traveler yet. I would look for properties that are thoroughly described and positively rated though. You're paying to stay some place, so if you ever have any questions it's the hosts obligation to respond to your concerns. If the host seems sketchy or something doesn't seem right, then keep looking. The one thing that did irk me though, as a host, was when that 'pushy' guest described above, had me drop the keys off to a studio out of my way instead of sending his fiance up to pick the keys up (my place is good enough for him to stay at, but he's worried about sending her to an unknown neighborhood?). He also left some private critique about how I left the clean sheets folded on top of the bed, and that it inconvenienced him to have to make his own bed when he arrived at 2am (and how another host had left him a toothbrush along with a made bed). If you want luxury, stay in a hotel is my philosophy.
So, if you do have certain expectations as a traveler, your responsibility is to communicate what you expect. Don't make any assumptions.