First and foremost, I have to say that I am pleased with my purchase of this particular Variable ND Lens Filter as someone (I'm a non-professional photographer) who has recently become interested in such kit/capability and don't wish to spend $100+ on higher-end equipment.

I see its greatest value in situations where time is of the essence and you are not able to fully control the situation - i.e., you don't have the luxury of time to set up shots using/switching fixed ND filters.

PROs:
*Doesn't appear to affect sharpness in any discernable way - The glass is of sufficient quality.
*Threading is tight and of good quality - Does not back-out when rotating the ND ring opposite to the way it is threaded on.
*Any color hue created in the lower ND settings is not too noticeable and is easily corrected in post. In several instances I've actually been pleased with the very slight hue it created and left it that way.
*The ND ring is tightly fitted but turns smoothly (no issues with it turning by itself when carried around all day - it seems to stay in place).
*Appreciate the plastic case it comes with to keep it from being damaged or collecting dust.
*Did not appear to affect my camera's autofocus and/or face detection.
*Probably obvious, but it's WAY faster to adjust this than removing and replacing fixed filters. This was handy when I was on a guided tour and in a vehicle where I didn't have time to change filters for particular shots, a common benefit probably to all variable filters of this type.

CONs:
*It does add a couple mm of length to your lens, but I suspect nearly all filters like this will do the same. If your camera/lens are already tightly fitted in your bag, having this filter on it could amplify the issue.
*No audible/tactile "click" when adjusting, although as stated before it's machined tight enough I never had any issues with it wandering from where I set it after several hours of walking around. More of a personal preference.

Summary:
For the money and its quality (at this price-point) it's sort of a no-brainer for someone like me who is/was interested in adding ND filter capability to outdoor photography, particularly landscape photography that can be troublesome during maximum lighting conditions (high noon) which can wash-out some contrast in your shots.