Pros
Excellent company culture, modern working (flexi time), sociable and friendly. Keen on personal development, put staff through training courses where there are skills gaps. Lots of responsibility/autonomy within engineering team and relatively flat management structure. Everyone helps out each other, roles are quite flexible (not pigeon holed down one task). Great location just outside of town, most of the team cycle in. Excellent and approachable management (all the way up to CEO) and strong focus of keeping the whole company in the loop. Exciting company to be in, continually expanding and innovating.
Cons
Not really a con but something to be aware of - nature of a startup is constantly changing requirements and priorities so be prepared to juggle multiple projects or stop working on a project and start a new one due to urgency required.