I have flex flares in the rear and love them. I can bend them all the way up to the body with my tire and they just pop right back w/ no damage to the tire or flare. have hit them will alot of force on many things and still look new and hold strong.
I'd vouch for their durability, but the problem I've had with mine is that they mount too low, and I was rubbing the tires all the time, started to chew up the tread. I moved them up about an inch and bent the ends out, but now the lines don't match, and I had to chop the back flares and mount half on the door. They are durable though, but if you're running much more than 33's I'm not sure how'd you fit them.
edit: If you were looking at clearances on the tires, the back is my 32" spare, not a 33x12.50 like the front.