Something smells rather fishy here for one simple reason you can't get something for nothing. This idea was dissected in the Register a few weeks ago with the comments ranging from 'a nice perpetual motion machine you have there' to 'TANSTAAFL'
Sorry but they might be able to harvest some power in the lab where conditions are easy to set up but it will not translate to the real world where the phone has to find the tower and if the device is harvesting power then the phone has to up the power output to get a good signal to the tower. Such a situation would make marginal connections impossible.

I'm a bit skeptical to say the least...but then I thought about regenerative braking in cars...perhaps that's a better analogy. It's just recovering "extra" unused waste radio waves.
Ok, not a perfect analogy, but you get my point.

This is like running a coffee pot from the heat it generates. It will not work.
If there is somehow some "excess" radiation being given off then find a way to reduce the excess, perhaps by identifying the direction to the nearest cell tower and only sending enough radio energy out in that direction. But "harvesting" the excess radiation and turning it back into electricity? stupid.

There is no way their 30% figure is accurate because if you look at what your phone is doing less than 30% of your phones battery is being used to transmit data so ever if you recoup 100% of that it would fall short of adding back 30% of battery life.
Even with data transfers phones typically download more than they upload so ideally phones don't spend that much time transmitting. Many people auto backup photos and videos but usually when connected to WiFi where the phone doesn't have to transmit at very high power to reach the wireless node anyway.
Even then I admit I am still skeptic. I think wrapping your phone in a case designed to absorb some of its transmit power would only require your phone to work harder to reach the antenna its communicating with. When your phone registers with the tower the tower probably reports its receive power to the phone so the phone can balance out its power level by transmitting with just enough power for the tower to receive it clearly without needing to transmit at excessively high power levels.
This case would essentially just create impedance (resistance) for the antenna to have to transmit hotter to overcome so there would likely be a net loss of battery life even if it seems like its recuperating something back.

Here's the actual math behind RF energy harvesters: http://drop-kicker.com/2014/06/ifind-rf-energy-harvesting-bluetooth-beacon/
Nowhere near enough to do what they're claiming it will do...
Hackaday also did a writeup on this: http://hackaday.com/2015/05/05/techcrunch-disrupt-charging-a-phone-with-its-own-transmitter/

Why not scavenge all radio energy such as that emanating from your own, connected cell tower? Hello Mr. Tesla!

Add me to the skeptics. Any efficient RF energy harvesting scheme needs to match the impedance of the radiation source, which results in only 50% efficiency (since half is lost in transmission). This is Jacobi's law (see http://en.wikipedia.org/wiki/Maximum_power_transfer_theorem). If you want more efficient use of power in a phone, put your phone where it has it's best access to a cell tower and use a BLE headset. Holding the phone interferes with the phone power transfer efficiency due to impedance changes induced by the hand (unless we can make our flesh more RF transparent). And, as a colleague pointed out at work, most people move their phones a lot during the course of a conversation adding to the dynamics of the problem. A better solution is to use beam steering as is used for efficiency in cell towers now and part of WiFi N and AC specs, but that requires multiple antennae and introduces its own set of problems. Maybe by the time we see Android or iOS version 25 phones. By then, all the tracking apps in our phones will be using close to 100% of the power and bandwidth anyway. Oh wait, they do that now.

30% could be true in standby mode. Would be much easier to change battery to larger capacity

If we cut to the chase this is a scam or the people trying to implement it are not so smart.