Pros
Lots of openness and opportunity to work on interesting projects - obviously it depends on the available pipeline of work but my experience is that Q5 works harder than any of their peers to listen to consultant's preferences and development when staffing projects. Opportunity to get involved in growing the business, very flat culture so 'junior' people can get much more exposure to more than in other places (senior clients, all aspects of project delivery and business development).
Cons
Work can be relentless - consulting is tough and demanding. Whatever the 'permission' it can be hard to prioritise other things when your team-mates are all up against it. Growth means the organisation is evolving and so roles and accountabilities can feel in flux... Can make it hard to predict what the future organisation will look like. Starting to put more structure around certain 'communities' (eg sector groups servicing certain types of client). This is natural but need to be very careful cliques don't form.