Until now, I really haven't tinkered much with diffusion filters as I mainly get the desired results with Photoshop and a brush. Keen to try these, I must say that I did like the results.

Caveat - I did most of my shooting with these filters using studio off-camera lighting and a range of modifiers.

The effect can be subtle or more dramatic depending largely on your subject and their skin/texture/features couples with how you like to light your scene/subject. A softer light on a subject with mostly clean/flat skin yields a nice dreamy effect that doesn't look too manufactured or plastic. With more harsh lighting even with the same subject, there is a softening effect yet is reduced due to lighting. I did stack the filters and was significantly softer under hard light but not as soft as the previous setting.

I did use these diffusion filters on a model I shoot very frequently and asked her to not do much to smooth out her face (she has little acne scars). I used both soft and hard lighting with the filter attached (went back to single filter) and the results were much more noticeable. I was happy to see that, again, didn't look too manufactured or over the top. I did double-up the filter and again the results were as expected with the amount of softening.

As far as highlights are concerned - this did aid in highlight control without squashing them out. Specular highlights were still visible yet muted down (based on a control image). It's not negligible but if you're not comparing to a control image, you probably won't see too much difference.

NOTE: To better enhance your effect make sure you have any in-camera sharpening turned off (which you should do anyway..LOL) and if you're tethering (Lightroom or Capture One) also make sure you're not sharpening! I am a big Capture One tether shooter and have found that CO does tend to add-in some sharpening on its own (compared RAW shot to card and same file saved to my computer in the CO file).

Other items - these filters did not cause any significant color shift (I meant to use my DataColor cards and.. well, didn't.. sorry..) or stop-down any light. The filters are very well made, the threads are nicely machined and finished, and come in a nice filter holder pouch.

While I may not use them for all shoots, I can (and have) used them for clients needing that little extra 'something'..