Pros
Challenging work; very intelligent coworkers, majority of engineers are Ph.D. Level; you gain hands on experience; long hours but it often doesn't feel like "work"; and good pay and benefits. Why so many negative reviews? If you're not happy or feel unappreciated, then you should consider working elsewhere and avoid start up companies. The nature of a start up is intense, more so in this industry. If you came here for work and life balance, or only for money and stock, then you came for the wrong reasons. We should be here because we want to do challenging work, the money and balance will show up in due time.
Cons
Not everyone is committed to the hard and intense work required; a lot of secrecy around projects; overly aggressive deadlines and goals.