Re: So, when are they going to reuse a booster?
The first reuse flight is for SES-10. Currently this should launch next month.
Actually it won't prove that landing boosters has value: it is just a prototype that shows you can. The real value preposition comes with the final iteration of the Falcon 9 design (Block 5) later this year. This design is "tweaked" with what has been learnt from the landed prototypes to make them cheap, safe and easy to reuse.
The SpaceX balance sheet will be the proof that landing boosters has value.