Pros
Because of the nature of this company (startup), you are expected to handle several aspects of your project. While this may be somewhat daunting, it is an excellent way to hone in your coding, design, and interpersonal skills. You will learn a lot about the software development cycle from working here, more so than any college course could teach you; truly invaluable experience!
Cons
The company's direction is not always clear. And while it may be highly lucrative to work here in a year's time, as of right now, there are virtually no benefits/salary to speak of. You will be getting stock options that vest after some time, but you will not be taking home any real cash. If this is a deal breaker for you, I would suggest looking elsewhere for employment.