No discussion of descriptivism and prescriptivism is complete without reading this article by David Foster Wallace.
Personally, I'm a descriptivist in all situations other than those where my fairly decent command of the langauge allows me to make fun of people with lesser linguistic abilities.
You know what's a funny word - pickelweasel.
Ha! Actually, here's a wonderful example of the benefits of descriptivism: "merdiwitted." I first saw it on RachelLucas.com, about two months ago. It's not in the OED, so I presume the guy who used it made it up. What a wonderful word. The meaning's obvious, yet understated; you could probably get away with using it in places where "shithead" wouldn't fly. I have to make a conscious effort not to overuse it.
Here's a good site for budding descriptivists. In fact, maybe I should blog a link. I always wondered what "fo' shizzle my nizzle" meant!
Just read the DFW piece. It was excellent. I'm still astonished at how much I like his essays given how little I enjoyed Infinite Jest (or the 30% of it I bothered to read).