Pros
The technology is quite cool and the end product is worthwhile. Being a startup there is no HR. Having only one project there are no timesheets! There is a certain amount of freedom to work how you know best without interference or admin. Management listen to staff who want to change things within reason. Obviously getting the product out on time takes priority and you will not get what you want if it would jeopardize that.
Cons
There are no documentation or quality control processes (or really processes of any kind) in place. This is changing slowly though and will continue to change as the company grows and as time allows. In the few months before releasing our first product everyone was working 60 hours a week and some managers were working 100+ hours a week. This has calmed down now (averaging the usual 40) and it probably will not happen again on such a scale, but you will still be expected to pull your weight when the occasion demands. Like any startup it is not impossible that it will go bust in 12 months! The CEO is based in the USA when all the rest of the company is in the UK. He is a typical salesman. Also the office is really really untidy!