Yes I have one. Well they offer two major benies depending on your need. First if you have a asshat of a local LE you get a trust and no more CLEO sign off, fingerprint cards or photo's. Second my reason for having one I have a stepdaughter and grand kids if my wife and I where to pass they would have to pay tax on every item as they are not direct relatives. Also when you make your trust ALL the COtrustees can posses the items and use then independently. IF you do not have a trust then you can not let your wife use the NFA item without YOU the person who's name is on the form 1 or 4 being there in control of the item. That's about the crux of it. I have my whole family listed as co trustees so the trust is in effect until the last one dies as we all own it together. You also list who inherits the trust when all the owners are gone they can transfer the ownership off all the items in the trust to them WITHOUT having to pay for a new stamp for each item they do have to do the paperwork though. You can do a Quicken trust for less then 100 bucks of pay a lawyer.
To this i'd like to add a couple of items. First only those on the trust may have any access to where they are stored, i.e. gunsafe, etc. for instance if your wife is not on the trust she cannot have access to the safe. Also those on the trust must be legally able to own a weapon.
It's a good idea to go through someone like Gun Trust Lawyer as the trust is reviewed by an affiliated lawyer in your state to make sure it conforms to state laws. Costs, but it's worth it as the BATFE accepts the trust but makes no ruling as to it's legality for your particular state.
Establishing an NFA trust through any means other than a lawyer who specializes in them is penny-wise and pound foolish--at best. Doing so carries bragging rights but not much else. Trust provisions are highly dependent on individual circumstances and are worded to cover individualized potential contingencies that laymen usually don't recognize as significant.
And to all these great points I'd like to add- that once you have a gun trust- you're a candidate for ATF's E Filing option. This will shorten your wait for from 7-10months down to approx. 3 months. You scan your FORM 4, and pay your tax stamp with a credit card.