I've got a set of Pypes cutouts on my '69. With my experience, knowing what I've learned since, if I had it to do over again I'd go with either Doug's or QTP. Be wary of any design that employs an electric motor mounted on a more or less square gearbox mounted off to the side. Here's a link to a "typical" image of one of these:
12V 1rpm Low Speed DC Geared Motor - China Dc Geared Motor, Low Rpm Gear Motor
These motors are sold 'off the shelf' by many suppliers and usually the gears inside that box are made out of plastic or some other non-metallic material and as such can be very susceptible to heat. I had one of mine fail when the gears inside were reduced to a puddle. It was replaced under warranty (after some difficulty), but recently I've had another problem that has twice (so far) required me to disassemble the unit and repair it. The small screws that hold the motor onto the gearbox housing "got loose" and pulled out of the thin stamped metal end cap of the motor. I was able to find some slightly larger screws and get them to bite into the motor housing, so for now it's working.
I've had mine on my car for going on three years now, with the aforementioned problems, and I've come to believe that probably any/all of them should be treated as a "novelty" item that's going to need periodic maintenance and replacement. They're lots of fun
but to be kind to all the manufacturers it's probably pretty difficult to engineer a product that's capable of long-term longevity in an exhaust system and exposed to road hazards/debris and be able to sell it at a competitive price. The way to look at these is to decide for yourself: is the enjoyment you're going to get out of having them worth the hassle of having to periodically "deal" with them?
99% of the time, when I "use" mine it's when I'm showing the car to someone or driving into/away from a show. I start the car, let it sit there and idle while the audience "appreciates" the lumpy cam, then I reach in and pull the cutouts open - everyone's grin get's bigger while they "oooh and ahhh" over it - then I close them, shut the car off, and start answering questions about the engine. I won't deny that it's lots of fun
- but I'm very careful to not run them on public roads where they might attract the wrong kind of attention and risk getting the car impounded.