I'm the first employee of BackType, a Summer '08 YC company. My joining the company increased the company size by 50%. The experience has been awesome, but I will say up front that being the first employee of a startup is not for everyone.
The best part of being the first employee of a startup is the total exposure to all parts of the company. I've learned a ton about product development, customer development, recruiting, and entrepreneurship. Additionally, I've met and connected with lots of other awesome people through the YC network. I've gotten all these benefits at relatively low risk for myself, as I still have a salary and a solid chunk of equity.
There are a lot of rules working at most companies. You don't even realize that some of the rules are rules until you work at a company with no rules. I'm talking about the most basic things like what hours you work, what days of the week you work, what tools you use, and whether you come into the office or not.
Because there are no rules, you have to be able to set your own direction and make decisions on your own.