First impressions deal with so much more than appearance. I can basically tell what a person is like from just 5 minutes of talking to them, as well as observing the way they treat others and their disposition. Granted, I have a degree in psychology...
Honestly, I form first impressions based off of low expectations. Once I further interact with that person, and they meet those ridiculously low expectations, I'll give them more of a chance and gradually increase those expectations based off of others who interact with them, and whether I trust those people in giving me an opinion of the person's character.
Yet, I can fully say that I'm at a point in my life where I don't need to impress or be impressed by someone new (unless it's perhaps a new company or boss, then I'll put that effort). And it's not that I'm saying I put forth no effort in the matter. But if I'm not being myself, or if I have to abdicate my morals and values in order to maintain pleasing this new person, it's not worth it when they constantly find ways to crush those expectations I have established for them to meet.