They don't have to do powered descent from orbit for this to be worthwhile. Just doing powered descent for the recovery of the first stage would be a huge win.
http://www.space.com/13140-spacex-private-reusable-rocket-el...
I'm sure they'll start with just the first stage, but you can't just do a powered descent - that would leave it in the middle of the Atlantic ocean. It has to be a return-to-launch-site maneuver and landing, which is what you see in the video linked to above. At the point where the vehicle does a 180-degree turn (referred to as a "death swoop" by rlv enthusiasts), it's going Mach 6 and is 1000 miles downrange. That's hard.