What may be getting overlooked with respect to reusable rocketry

A few points to keep in mind about SpaceX's recent achievement in landing the Falcon 9 first stage rocket and recovering it intact.  A comparison with the Space Shuttle's main engine may not be apt for a number of reasons

1.   A different fuel that burns hotter in the Shuttle main engines.
2.  The Shuttle's engines burned all the way up, whereas this is only a first stage.  The return to launch site burn only took a few of the engines.  The burn was a lot shorter. 
3.  Besides the Shuttle, there were rocket planes flown in the fifties, and these were reusable.  How well did these hold up?  The point is that these engines may be more durable than commonly believed at the moment because of the Shuttle experience.
4. The Shuttle engines had to be rebuilt because there was no engine out capability.  A loss of a Shuttle engine meant an abort.  The Falcon 9 has an engine out capability.  No need to abort a mission because of a failed engine.  They can afford to push the envelope a little more.
5. The SpaceX people may already have a pretty good idea of how long an engine could last from test firings on the stand.  It stands to reason that they wouldn't spend a lot of money recovering a first stage rocket that wouldn't be useful at all.  In other words, they have to believe that they have a good chance to fly again, and that it isn't based upon wishful thinking.

I'd rate their chances at better than 50-50 at this point.  You don't really know how much better than 50-50, but it is definitely better than 50-50.