| Today the answer is vintage hype. They are rare and "holy grail" units among covetous guitarists and producers. That said, MXR's blue-face flanger and pitch transposer were, for many years, simply the best-sounding units of their category. I am not a fan of the phrase "the best", but again for decades almost nothing could touch those two units for tone. I'm using the past tense because (a) I believe some modern "boutique" effects have caught up to the tone quality of these MXRs, and (b) the MXRs are deteriorating from age. They are getting noisier and noisier, and more prone to failure. I owned two Pitch Transposers and a Flanger/Doubler, and I used to swear they were going to the grave with me. But I finally let them go when I realized how audibly they were deteriorating, and that I couldn't afford to have them completely overhauled.
In retrospect I should have kept one of the Pitch Transposers and saved up for an overhaul on it. There still isn't anything quite like it on the market. But the Flanger-Doubler, there are others around which sound pretty good, so I don't regret selling it. | 
