First off, according to the forum rules, this is a legitimate post/question.  Second, according to the site (and it's a reputable site) these modifications are perfectly BATF-legal.

My question is - has anyone ever tried these?  If so, how do they work?  Are they really legal?  If so, why?  

If they are indeed legal, of the 4 devices, which one would be the best to use?

[url]http://www.firequest.com/catalog/trigger_system.html[/url]

The reason that these are legal, is that they all suck.  Sure, some can get these to simulate FA, but some can do that by bump firing without these things.  All will provide poor accuracy, as most of your attention will be focused on getting them to work, rather than being on your target.  The gat is the only one that will allow a firmly shouldered weapon, but even then, your trigger hand has to crank it.

IMO, forget about it...

So basically, they are mechanical devices that pull the trigger "really quickly"?