I've shot quite a few machineguns over the years, probably 30-50 different platforms in various calibers, ranging from AM-180s to M2HBs. I had the chance to shoot the original Akins Accelerator before it was shut down by ATF, and I've shot probably half a dozen SlideFire equipped ARs and AKs.
Plenty of people hate on the SlideFire SFS because of perceived safety aspects, or legal issues, or simply because they find it unseemly. From a technical perspective, the SlideFire works just fine if you put it on an in-spec rifle and operate it correctly. I've run them on 5.56, 9mm, 7.62x39, and 22LR AR-15s, and both 7.62 and 12ga AK platforms. It helps to have the rifle configured ergonomically to work with the SFS, so that it can be held and operated properly. From a safety perspective, the SFS doesn't do anything that a rifle isn't already doing - you're not going to have some sort of magical out-of-battery detonation by pulling the trigger "too soon". Anyone with a basic familiarity with semi-auto firearm design should be able to understand why that's just not possible.
From a tactical perspective, a direct comparison between say, a Colt R0977 with selector set to full, and a SlideFire SFS-equipped Colt LE6920, you'd find that most shooters could probably get a higher percentage of rounds on target with a magazine dump from the R0977 select-fire rifle, with all else being held equal. The ability to selectively discharge a certain number of rounds in a single burst is going to be equally easier with the R0977, since there's less movement required in a trigger finger versus an entire arm. That being said, an experienced shooter with the LE6920 who's practiced using the SFS would easily out-score a relatively inexperienced individual using the R0977.
It occurs to me that a fair amount of the inaccuracy perception may also be related to the fact that most SlideFire owners and shooters have never really had any experience with automatic weapons before, and probably aren't receiving any instruction in their use when they first shoot the SlideFire. Essentially no one ever learns how to shoot a legal automatic weapon by simply picking one up and blasting away - there's almost always other individuals around who already know how to shoot it. The 86 ban ensures that owners of NFA-registered automatics have a vested interest in protecting their property and keeping it safe, so there tends to be a rather higher level of safety and knowledge in the vicinity of NFA machineguns. The same cannot always be said for the SlideFire, but ultimately you cannot fault the product nor the concept for that fact.
For what it's worth, I know several NFA collectors who still find the SlideFire to be plenty enjoyable, and who wind up shooting it nearly as much as their registered MGs. I've had guys with collections worth hundreds of thousands of dollars tell me "Why should I buy another $20,000 machinegun when this $300 SlideFire is just as fun?" And ultimately, they do have a point. When NFA ownership is out of reach for the majority of shooters, the SFS is a tempting and enjoyable alternative. It may never be as valuable, effective, or sexy as owning the real thing, but it's still fun for most. Deriding that fact is ultimately nothing more than gun snobbery, like it or not.
