Some of the comments are interesting, reading through the math (even if I don't fully understand it).
Some of them, and the cynicism of some people, are really disturbing.
It's not unreasonable to be cynical in some cases. This is one:
the experiments make an extraordinary claim - that of a totally new source of energy - yet have decided not to publish their data in the normal way, but by press conference.
they propose no plausible explanation for their effect, nor have analysed the device after use to see if their is evidence for the existence of their explanation.
their work has been rejected from scientific journals, for reasons unknown. However, it's probably because the descriptions of their analysis did not provide enough detail to be confident that measurement error or fraud were unlikely. (e.g. it's very easy to mismeasure certain things - electrical power being one of them - hence the profusion of 'electromagnetic overunity' devices working at high-frequency/high-voltage, where measurement requires exceptional care and special equipment)
The work has subsequently been published in a 'scientific journal', but the 'journal' is in-fact self-published by the experimenters and not peer-reviewed.
The type of 'partial' public demonstration given is quite common for hoaxes. I've seen it many times, where some 'magical' device is placed in a test rig. Engineers are brought and allowed to cursorily examine the test rig, but not the device itself. Often the examination is purely a description of what things are, not an explanation of what they do - it's surprisingly easy to mislead people when they only have limited information (e.g. looking inside a box and seeing devices, but not actually telling them, or letting them check, what the devices actually do) This isn't proof, or even evidence, of a hoax - but it is a sign of lack of openness.
Andrea Rossi (the lead experimenter) is an ex-con, having served jail-time for fraud. This does not mean that he is being dishonest in this case. However, it does mean that he has experience in deception, and has learned a hard lesson in how not to get caught.
Rossi has also been involved in a scandal regarding one of his earlier inventions - a technique for refining trash into oil, in order to avoid dumping garbage in landfill. This worked fine, but the the process concentrated all the toxins - heavy metals, etc. into a waste product. It caused a major scandal when it came out that this toxic waste product was simply being relabelled 'garbage' and shipped off to the municipal landfill.
The claim that they are delaying releasing information until the patent is granted is doubtful. Once you have filed the patent application, any duplication of the work will be proteced by the patent, once the patent is granted. This allows inventors to publish or sell publicly devices which have not yet completed the patent process, without risking the patent protection.
This story has so many 'red-flags' for a scam, it's amazing that it's got as much attention as it has. Still, if they are onto something, then it looks amazing, and I wish them luck. Realistically, though, I'm not holding my breath.