What it is:
The Hell-Fire is a small metal gizmo that clamps to the triggerguard behind the trigger of your semi-automatic rifle or pistol. It has a "finger" that presses against the back of your trigger, and the pressure exerted on the trigger is adjustable. The net effect of this is to increase the force that attempts to return your trigger to its forward position.
What it does:
It helps you to fire your gun at "full-auto" rates, though you are still pulling the trigger each time, so it's legal in the US (it even comes with a copy of a letter from the BATF stating that it's legal). I do not know what other countries it might be legal in.
How you use it:
Attach it to the gun, then hold your gun out front with your weak (non-trigger) hand. Put your finger on the trigger and don't let any other part of your strong (trigger) hand or body touch the gun. You are now supporting the full weight of the gun (and aiming it ) with your weak hand, and your trigger finger is touching the trigger. Now, push the gun forward with your weak hand, and the gun will fire. Keep pushing with the weak hand. If everything is balanced, the recoil will move the gun back enough to re-cock the trigger (with the help of the Hell-Fire attachment) and the pressure from the weak hand will move the gun back forward, which will cause it to fire again. Typical fire rates are on the order of 10 per second (depends on the gun, of course). It will take some practice to get the hang of keeping your muscle tension right to keep the gun firing. However, with some practice, you will be able to reliably fire short bursts or empty an entire magazine.
Note that many of the ads for this product show people holding onto the gun with their trigger hand. This is misleading, to say the least.
Problems with the Hell-Fire:
The obvious problem is that it can cost several dollars a second to fire a Hell-Fire equipped gun. The less obvious (until you've actually used it) problem is that it's nearly impossible to aim the gun. Hitting a human-sized target reliably at 75 feet is quite difficult (much, much more difficult than with a real "full auto"). And it's not easy to transition from "full auto" to aimed fire or vice-versa. Additionally, full-auto firing heats up your gun very quickly. It's possible that this could damage your gun.
What guns it works with:
MAK-90s and other AK-47 derivatives, Mini-14s & Mini-30s, AR-15s and clones, SKS's, M-11s (all flavors and calibers), and scads of similar guns. For pistols, you will have to have a barrel shroud or fake suppressor to give you something to hold onto on the front of the gun.
Note that none of the above shoot .22 LR. The light recoil of this round will make the normal Hell-Fire difficult to use, though there is now a version available for the 10/22. I have not tried this new version. However, if you have a .22, there are various "cranks" and other gizmos that provide a similar effect, and probably do a better job since you can press the gun against your shoulder (and thus aim).
Is it worth buying:
That of course depends on you. It's certainly fun for an afternoon, and will definitely impress your friends, but true "rock 'n roll" it ain't. It may make an interesting collector's item, what will all the media attention that it has received.
The lawsuit story:
The maker, Hell-Fire systems of Olathe CO has been put out of business by a lawsuit filed against it over the shootings at a San Francisco law firm. Survivors and the family of at least one of those killed have sued each company that made anything the killer brought with him, including the Hell-Fire gadget and the Tec-DC9 pistol. However, it appears that another company (Frank's Center) bought the rights (?) and is now selling the things.
Where to get one:
The only source I am aware of currently is Frank's Center, P.O. Box 530, Nevada, MO 64772. Price is $29.95 for the "normal" one, and $19.95 for the version for the 10/22. Phone number is (800) 666-9190. Lower-priced clones of the Hell-Fire have been available at different times over the last few years.