I owned a Helen for more than 3 years. I'm 100% sure that it improve sound quality at audible level. After introducing Helen into my system, the image became more stable and refined. The bass which used to be loose now is tightened. The general impression of change is it makes sound clearer and more realistic.
That is my experience on a headphone system, some Helen user said the improvement is more obvious on a stereo system. It has really good reputation among the users, I heard people complain it's price is too expensive, but I never hear anybody say that it doesn't do as advertised.
Many modern DACs have built-in phase locked loops(PLL), Helen is just a external one. Well designed PLLs improve sound quality by attenuate jitters, it's a proven fact. There is nothing to doubt the theory that Helen based on.
My opinion on Helen is: It is a great idea, it works well but it's not for everybody.
A big CON of Helen, it takes 12 secs to lock to new sample rates. Within the first 12 secs after changing sample rate, you hear only silence. If you 1. use CD as digital source, 2. listen to music files that are mostly one sample rate or 3. upsample everything to one sample rate. In these situations, you can avoid this issue and only get the good side of Helen.
I listen to both Qobuz and CD music, I set up my system in a way that both paths can benefit from the Helen. I upsample everything from Qobuz to 768Khz via HQPlayer and everything played by the CD is always 44.1Khz, so Helen is perfect for me.
To give you an idea of how much Helen would improve the sound quality, I would say that, the improvement brought by HQPlayer upsampling is more obvious than Helen, despite that they improve sound quality in different ways. But Helen makes more obvious changes than swapping interconnect cables.
Does it worth 1600USD? It depends on how you set up your system. If you have a low budget system, I would say use that money to upgrade your DAC first. If you already have a mature Hi-end system, the price of Helen might not be a big deal for you, Helen might be worth trying.
If somebody insist that Helen is snake oil, I would say that at least with its price tag, it only targets at higher budget audiophiles.