[Aluminum] If you're looking at this filter you likely already know what it is for, and sure enough it does exactly what Tiffen say. Optical resolution drops down but contrast loss and halation are kept to an absolute minimum. Strangely, if you look at Tiffen's triangle of diffusion diagram you'll see they put this filter halfway between resolution reduction and halation, implying you will get some soft highlight spreading. You absolutely do not and I don't know why that particular diagram suggests it when all their text and videos say it doesn't cause halation and, as I said, in use it certainly does not.
Allegedly on wide-angle shots or when using very small apertures you may see the black dots in the image. I can't say I've noticed this myself, but I only use this for shoulder-up shots, typically 50mm, often at fairly wide apertures, so I'm not in that 'danger zone'. If you want to shot wide and stopped down, maybe take caution and check out the Digital Diffusion/FX instead, which is meant to cause the same resolution drop without the black dots getting in the way.

I've had some limited experience with some Tiffen filters having poorly-cut threads, but otherwise the construction quality has been fine. Can't guarantee you'll get a faultless one but the fault rate, in my experience, has at least been very low – about one out of twenty filters having an issue, and even then, not one severe enough to stop the filter from mounting and functioning.

[Aluminum] I bought this filter for HDSLR video use. The 1/2 grade provides a very subtle diffusion effect with minimal contrast loss. Ultimately, it takes the edge off of HDSLR video and provides a very close alternative to that elusive film look.

[Aluminum] I bought it for video to have smoother images but I discovered that I also like it very much for portrait photography to smooth skin without having the dreamy halo effect. To my taste, the effect is very subttle, though...

[Glass] Most photographers use at least one diffusion filter for portraits, whether a professional whose clients need to look great, or the rest of us who want to flatter Granny at her 90th birthday celebration party. I have kids with iffy complexions in their adolescence and didn’t want to rely too heavily on Photoshop for the fix.

For use on my Quattro DP2, this filter does a fine job of holding small details like eyelashes, yet blends blemishes well. (One caveat: a large blackhead or dark pimple may be accentuated, depending on circumstances, so a white diffusion filter or dab of concealer could be needed.) Since the eyes and lashes/eyebrows hold their detail portraits can be very flattering with this Tiffen product.

It looks odd by itself as it appears to be glass with a pattern of little black marks all over it, where I more or less expected some type of black screen or grid. Not being an optical engineer and as well as the filter works, I chalk it up to FM.

Also, I prefer to use Tiffen products in general as they have always be good quality, competitively priced, and made in the good ol’ U. S. of A.; what’s not to like?

[Glass] This Gold Diffusion FX 1/4 is a very nice filter to take the edge off digital. A golden glow is added with no or very little halation and I use it at outdoor parties and blond portraits.

[Aluminum] Probably my favorite of this series - I have the series in 72mm. Nice to use on any blonde/fair skinned person. Afternoon shots in overcast days in the #1 is good as well!

[Glass] One of the best portrait filters I have ever used.Skin tones are softened beautifully,small blemishes and wrinkles disappear while leaving everything sharp.

[Aluminum] I own two of these for two camera shoots. They produce a nice subtle softening for skin and reduction in overall contrast. Not too much, just enough to take the edge of digital cinema cameras.

[Aluminum] The Tiffen Black Diffusion/FX 3 filter produces the effect I was looking for. It's well made with a nice finish and threads easily on my Nikon glass.

[Aluminum] I bought the filter for two specific shoots. Both went well and I have no complaints. I used the filter on my 77mm Canon 17-40mm L for shoots early morning and late evening in the Fall Colors and the results were fantastic. Would recommend to others. Didn't find anything I didn't like.