I would disagree slightly with Frank (Wookie) above. Admittedly I am a CCR zealot but I also have (and teach) a Kiss GEM SCR and spent several years diving and teaching other SCR models. I was never very fond of the active SCR rebreathers such as the Dolphin. With those units there was a constant flow of nitrox gas that was not keyed to your metabolism or respiration. Also, you could kill yourself by forgetting to turn on your gas supply.
The GEM is a passive SCR and, in my opinion, a much better unit. It also uses a nitrox gas supply and you will have about 3% lower FIO2 in the breathing loop than is in your cylinder. However, approximately 1/3 of your breath is discharged with each exhalation and this gas is made up by your next inhalation triggering a standard 2nd stage regulator (an Aqualung Micron) to add the gas to your breathing loop. The advantage of this system is that it is keyed by your respiration and is not the constant flow of the active SCRs. Also, if you see discharge bubbles and you hear the 2nd stage firing with each inhalation, you know the system is working. Obviously you still must monitor your PO2 like on any rebreather but the GEM is a very simple and robust system.
As to where does this system fit into your diving. If your goal is deep and technical diving, I would agree with Frank that you should just go the CCR route and get a system that can "grow" with your diving. However, if all you are looking for is "gas extension" in recreational diving, the GEM fits the bill quite well. This system is especially good for the "air hog" who is always calling the dive when his or her buddy still has lots of gas left. It is also good for smaller people, frequently women, who would like to be able to carry smaller lighter cylinders but still have plenty of gas supply. Finally, while there are bubbles, they are minimal and it does make a difference in getting close to certain marine life. Then there is the avoidance of "cotton mouth" by breathing warm, moist gas.
Remember, however, this is simply "gas efficient nitrox". You still have the same concerns with MOD and decompression that you would have in open circuit nitrox diving. So, if the advantages make up for the increased cost for you as a diver, the GEM is a good unit. If not, stay open circuit. And if you are looking at deeper more technical diving, at least in my opinion, consider going CCR and not SCR.
Just my 2 cents.
Doug
