I went down on wet pavement once right after a rainstorm and got bad road rash on my elbow. I had just bought a new expensive biking rain jacket, and actually lucked out that somehow the sleeve pushed itself up and I got road rash (which healed on it's own) rather than making the jacket useless by tearing up the sleeve.

New bike, stock tires with the bike, slightly aggressive riding (though not overly so). After that I threw out the tires and put on gp4000's, and it was very obvious in riding that they had a lot more grip on wet pavement.