Pros
I joined Box.net right after graduate school, and it's been an amazing experience for the past ~2 years. This is a place where there's too much to do, so I never got bored. It can be chaotic, but in the process I have the opportunity to be involved in many product launches in a small period of time - more than I saw at any company where I interned. There's also plenty of support from everyone at the company, who share a high level of enthusiasm, motivation, and love of the work. It's really a challenge, the work is diverse, and the team is diverse. The founders are very young but brilliant, and brought on more experienced talent as the company grew - so you see a variety of different personalities that work together. Then there's the benefits - I work hard, but the company makes it up through a flexible schedule. They also provided extra perks comparable to those of larger companies.
Kontras
There's the standard challenges of a startup - having too much to do with the resources you have, and dealing with constant change (most recently from the sudden growth in the company's size). I wouldn't actually consider this a bad thing, just uncomfortable at times, but great for experience.