Pros
The "base product" is really only used for sales. After a client signs on the base product is heavily customized for that client's needs, which makes for opportunity to do something new fairly regularly. Along with all this greenish-field development, Jenkon is usually willing to try latest techs, stacks and frameworks on different clients, so you can get exposure to a lot of different things.
Cons
The downside to customizing code in such a way is that any given customer's branch isn't recognizable as the base product any more, and rarely are changes or bug fixes in one customer usable with another customer. Which means Jenkon is essentially a consulting company, but act like they are selling (and supporting) a product. I am not saying that consulting is a con, but Jenkon's hybrid consulting/product approach has inherent difficulties to support all that customized code. Upgrades were a nightmare because all of the customers' features were essentially rebuilt off of the latest "base product" branch, essentially meaning customers had to pay again to implement the same thing. It might be that I'm not cut out for consulting-type work, but I found it very hard to be proud of my work without the type of direction that you usually see in developing a single product.