I have a vintage MXR Envelope filter I acquired in a trade back in the 80s. Not even really knowing what it was, I remember plugging it in for the first time and thinking "oh, Jerry Garcia." I was a relatively new fan at the time and getting that pedal really started my interest in exploring GD music as a musician.
I've bought and sold many envelope filters over the years and I keep going back to the MXR (it's on my current pedal board). I love the simplicity of dialing it in with the two knobs.
I also have one I bought a couple years ago off Ebay that is branded as "Newkirk." The builder claims he uses NOS MXR PCBs to build these (if I remember correct, Analog Man's original run also used the NOS MXR PCBs). It's true bypass, has an on/off LED, and has a 9V adapter, all things the original did not. Pretty true the original, I find I have the knobs a bit higher then the original to get the sound I'm looking for.