I have a larger and a smaller Nightstar. Both are sealed (said to float) with magnetic switches. There is what some would say an optic (magnifier) over the very blue, very weak emitting LED. The bigger one is brighter initially but both go dim pretty soon. I checked out their website a few months ago (appliedinnotech). They sell different models than what I bought, maybe 6 years ago. After the initial shaking, it does not take much to bring them to their full brightness, but if you leave them alone for many days, you will be at square one. To me, having batteries in a shake light does not perform their function. The manufacturer has said that if the capacitors are kept fairly cool, they could last more than a decade (I think). In the latter 80's, I worked security and tried different lights. I tried a Japanese, then a Russian, and then a Chinese dynamo powered light. That is, if you stopped squeezing, they soon went out. They were all using incans and the plastic gears on all of them failed after not that much regular use. My boss at the time chided me for making so much noise because of the light. I had a backup light of course. Now maybe those more modern dynamo lights that you crank to wind up a spiral spring mechanism and a flywheel does the spinning might be more viable than a shake light. Since it has gears and such, it seems the shake light might last longer but I do not know. Comparing a Nitecore TM26 XML lowest setting (3 lumens) to the biggest (Nightstar 3), I'm guessing maybe 5 lumens, but as you said, it goes to sub lumen, I would say, after a few minutes. The smaller (Nightstar CS2) is maybe 3 lumens at maximum. I just think they may come in handy in dire circumstances. Perhaps a loaner in times of need. No batteries, no gears or springs. Maybe I've been duped.
Years ago on Saturday Night Live, the guy who now sits in Johnny Carson's chair did a skit with a shake light, copying the act of masturbation. Not a very funny skit and I can tell you the proper method to charge the capacitor is to hold it horizontally, and not shaking so hard as to slam the repelling magnets on both ends.
Now, I can only hope a moderator does not summarily delete this post for such apostasy.
I'm not sure if they still make the nightestar, and a couple of years ago they were a bit more expensive than other shake lights. I bought a couple shake lights, none nightstar, but the capacitor ones were as bright as they were going to get (not too bright) and dropped off pretty quickly. The others had NiMH batteries in them, but it took a LOT OF SHAKING to charge those babies up.. certainly more than 30 seconds.
From my limited experience, it seems that the dynamo (crank) type takes less human energy to create stored energy than the shake type.
I'm not sure what they are advertising, something like 20 minutes of "useable" light for 30 seconds of shaking. Maybe reppans will find it useable, but I'd bet they are talking sublumen at the end of a 20 minute run.
I don't want to slam you on this, but question whether it should really be listed as an under-rated light. Do you have a nightstar? Is it really MUCH better than the others, and since they don't list the lumens level of output, can you give us an estimate?