I've used NS a few times for couches to sleep on. Just post in the regionals and someone pretty much always responds. My experiences have been good and I trust a skier more than a hippy.
That being said, my friend and I hosted quite a few Couchsurfers from the website this summer. Mostly euro's hitting up Yosemite. I thought it was fun, they'd usually make us dinner and clean up really well and in return we got to show someone an awesome time.
I say it's totally worth it.
I'm going to france/switzerland in a week and will be spending a night or two on couches to save a bit of extra money. Totally worth it.