Peeling from the PDW thread, I think the little Rattler is beyond cool and I get the attraction both as a defensive weapon and the non-NFA travel legal PDW side of things.
As someone who can remember the dozen or so times in my life that ATF has approved something in writing, and then completely changed their mind later (e.g. bump stocks, Akins accellerator, Streetsweeper 12ga, open bolt semi-autos of all types, etc. etc. etc.), I remain skeptical that pistols with side folding braces will remain legal forever. But I hope I am wrong and they opened the floodgates on that sort of thing forever.
For a compact travel friendly PDW'ish thing, but down the shotgun path, I have owned an old Stevens 520 and a 620 (it is a takedown design) that I set up in 18.5" riot gun configuration and 12.5" LOP, as well as a takedown Winchester M12. The advantage of all those is I can take them apart and have something transportable in a package almost as small as the little Rattler. But forget quick deployment.
I find the old over folding versions of the 870 stock to suck for shooting and the LOP was way too long for me. As for the pistol stock only shotgun (or "firearm") variants, I find them all to be an abominable shooting experience and not likely to improve any aspect of my marksmanship over and above my pistol.
I owned one of the Choate side foldering stocks with the pistol grip, it was a wobbly horrible POS.
I think the bullpup configuration shotguns likely have promise but I am not convinced any of them are truly reliable yet, or at least one that is not too heavy and bulky has proven reliable.
I am not ready to trust the bullpup 870 conversions, since I don't have a trusted source to tell me those really work well. Although that would certainly seem to hold promise.
What I wish someone would make is a really a decent practical side folding stock for my 18" 870. Something like the magpul stock but side folding. Like the LAW folding adapter for an AR but it fits the Magpul 870 stock... I don't consider the clones of an AR carbine stock designed to fit the 870 as a decent solution...tried one, it sucked for me shooting it.
I know that a 26" folded 870 package is still too long to fit the ideal PDW profile many think of, and since it is not a rifle or pistol cartridge it is not a real PDW in the classic sense. But it would not be very heavy, would fit any suitcase I own or most gym duffels, some backpacks. And that 870 level of reliability and shootability at least. And for certain as legal of a transportable firearm as anything that size could be. If somebody knows of one such adapter, please post it here.