Re: Don't call it "re-used"
"has never been done before"...
... except for the space shuttle boosters, and the shuttle itself. And Blue Origin, if you want to count suborbital hops.
The question is: Will it actually be cheaper? The shuttle itself cost a fortune to refurbish between launches, which was one of the things that made the shuttle uneconomical. We won't know whether SpaceX can refurbish cheaply until there have been a few launches with reused rockets and we can see what they're charging. The refurbishment cost will also probably go down over time, as they learn what systems don't need refurbishing every launch.
They said the first one that came back was going to get torn apart and examined and tested to destruction; one of the later ones has had a refurb, probably a fairly major one, and is about to fly again. Musk's stated goal is just to refuel them, time will tell if he can pull it off. I wish him all the best - it will be great if he can.