Open X is definitely a good tool to get you up and running cheaply, but isn't as full featured as some of the other ad servers (ie. DFP)

Keep in mind that the entire display ecosystem is already on / moving towards an exchange-based model (rather than a network based model). Most networks now buy a lot of their inventory directly from the exchange.

So if you're looking to start your own network, you should be thinking about building it on top of the exchanges, and you should also have some unique offering: expertise in display / optimization, unique data set, etc.