To me, it's not "very much like [you'd] use US dollars" at all. You didn't actually buy anything, did you? You used it to trade more bitcoins. I mean, if people were just pushing around dollars, exchanging them with each other, and not purchasing actual goods and services with them, then maybe it would be the same.

On the other hand, my knowledge of all this is pretty superficial. But the one thing that sticks with me that the naysayers keep bringing up is just that: no one (or very, very few) is actually using bitcoins in any useful way other than trading them with other bitcoins.