The use of contractors will continue to grow for the following reasons:
1. High-tech: for tech contractors the government can no longer compete on the open market. High-tech contractors are rare within military ranks today, and are rare even within civil service.
2. Perception of decreased cost: although the short term cost of a contractor is high, the government doesn’t have to worry about keeping on a long-term employee, especially one that requires retirement. (I use the word “perception” because there’s still a debate about whether contractors are cheaper than regular employees.
3. Contractors vs. military: this administration does not want to increase the end-strength of the US military so it has to use contractors for all kinds of work, from info-tech to security.
4. Contractors create less of a PR problem if they are killed or captured: using contractors for all kinds of security duties means you don’t need to put soldiers. And if a contractor is killed or captured, it’s no big deal to the public at large. (Remember that there are still three US contractors being held by the FARC in Colombia – how many Americans know or care? But if they were soldiers, politicians would make an issue of it periodically.

I don’t think the issue of “success” or “failure” is a big factor. In the end I think cost, public perception, and politics will trump “success” or “failure” because there are no standardized measurable criteria to make a valid determination – people have opinions generally based on anecdotes. In situations like that politics, public perception, and cost drive all.