I've gone down.. uhh.. 5 times? Once was sand in a slow sharp turn, the bike was simply not there anymore. It was actually a pretty odd experience.
The second was wet concrete, not the textured stuff, the slick as hell when wet stuff. Again a low speed turn, wet concrete is as slick as ice. Bike was simply gone, felt the rear tire break that time and said oh well and let her go.
Third was due to a f-150 cutting me off and taking my front end out while leaning in a slow left hand turn, nothing I could have done there. In that case I actually had time to react I suppose, I made sure it was going to force me to lowside, got my left leg ready to get out from under her before she slammed into the ground, and slowed enough not slam into the back of the truck and end up in his bed.
Put on diablo strada's at this point. And had a full set of beat em up track plastics.
The fourth was oil at the track in a decent hairpin at around 55mph. Again the bike was simply gone no time to react. Spread my arms and legs out and slide to a stop. Bike slid down the track a good 20ft no damage to either of us. Didn't even rash my suit.
Fifth and most recent (about 4 months ago) was my own fault. I had been practicing endo's the whole week (in a closed parking lot with police permission before the flames come my way), went back to regulaur tire pressure and grabbed too much brake. The ground was uneven and came to a sharp point, tire left the ground locked up midair, hit the ground and washed out. **** happens saw it comming, almost saved it, let go of the brake but it started to headshake pretty bad so I bailed to avoid highsiding. Was less than 15mph.
All in all I figure crashing is part of learning. As long as you don't crash the same way twice you'll come out wiser. Most the time you won't have time to react, but if you do it enough can minimize the damage to you and your bike for the most part. Most of all don't panic, know what's going to happen, try to save it if you can, but most importantly remember it's cheaper to fix your bike that it is to fix you.