I'm against them, largely because artificial sweeteners give me migraines. Frankly, I'm against putting anything "artificial" in my body, but I DO live in America in 2010, so it's a little difficult to get around.
But do I think diet sodas impede weight loss? Probably not. I think it's a choice. Be aware that like Matt said, some people do find they have cravings with diet. But if you don't, well then great.