Pros
You can carve out your own space within this company. It can be a tricky path but if you contribute to some successful projects you can build your internal brand and thus your own business within the company. A lot will depend on the relationships you build with colleagues in control of budgets. Most people are paid a base salary along with bonus system similar to billing hours/commission.
Cons
Since virtually everyone is paid on a commission system, you are extremely reliant on the sales team who are the managing partners of the company. The integrity of some of the MP's can be questionable and they build their own fiefdoms within the organization. If you need any structure this is not the right fit. For example there is no time off policy...take as little or as much as you need. It sounds interesting at first but in practice is very difficult because you are always looking for work internally. This biggest worry of all it that senior leadership is not very strong, a clear example of the dangers of nepotism at a private company.