Check out my article on the Chief Outsiders page re the tenure of CMOs in SaaS technology companies. It's an interesting datapoint when you need to decide between an Interim CMO or a Permanent Hire.
The difference between making money with SEM/PPC campaigns, and throwing money into a bottomless pit is how you execute the final 10%. When you outsource this to you should especially mind full about this. When you invest in Pay-Per-Click Advertising, on Google (Adwords), Bing or Social media like Facebook, Twitter, and LinkedIn, it's not about doing everything right. It's about doing everything a bit better than your competition.