If you are getting into long exposure photography there are a few things you need to have: a sturdy tripod, a good wide lens, a remote shutter release and, unless you want to photograph only at night, a good Neutral Density filter.
For years I've been using a cheap but quite good Cokin P-System with Cokin ND8 filters. They are cheap, they add noise and decrease sharpness, but at the end they do a decent job for the price you pay, and for a while a ND8 filter was enough for me. Obviously I was forced to use them only in the golden hour, when there's still enough light but not that much, and in that case you can bring home great shots even with a simple ND8 filter, or, if you want, you can stack two of them, so you get 6 stops of light reduction, but the price to pay is very poor quality and a very heavy purple color cast, which in some case it might be fine, but most of the times is not.
So last november I decided to invest some money to get a better ND filter, and a few months early I was well impressed by the filter of a good friend of mine, it was (nominally) an ND400 (which is 8 stop light reduction) at his maximum, and a light ND4 at the minimum. I'm specifying maximum and minimum because this is a variable filter, which in many cases it's a cool thing.

We are talking about the Fader ND Mk II, a variable ND filter made by a (guess) small company called Light Craft Workshop.
Reading the specs it looks a really cool toy for a photographer. As I said it's a variable filter which starts from ND4 (-2 stops of light) till ND400 (-8 stops of light), it's the classical screw-in filter, so no crazy holders like the Cokin P-System I had, which it's great, and it's very portable and easy to use.
So when I bought it last november I was quite excited to use it on the field and take some long exposures in daylight, something I couldn't do with my previous filter, and I was looking forward to go out and test it.
What was my first impression after my first shoot? Bad, really bad.
At maximum "power" it was absolutely unusable, at 3/4 was barely usable, and only at half power it gave me a good shoot, but at that point I was far away from the 8 stops reduction I was dreaming of.
Why unusable? Well the picture below will explain how, and bear in mind, I did set it on the max marker, I know that you can archive the same "effect" going after the max, but I always double checked if I didn't move the filter by mistake.

12mm f/16 1.6 sec ISO 160

As you can see there are a few darker areas, the effect is way too uneven, and this shoot for me is far away from being useful.
This effect it wasn't visible on my friend's sample, so for a while I thought that maybe I just got a faulty sample, I was thinking to go back to the shop and ask for a replacement. I tried to used it many times, to see if maybe there were something effecting this "X" shape effect, and in fact I noticed that the longer was the focal length, the less the effect was visible. For example, at 10mm is usable only at half power, while at 20mm you can use it at 3/4.
Then another confirm that the focal length was the cause of my problems came when I bought for less than 10 euros a kit of step-up rings, so I was able to test this filter on other lenses, as only my Sigma 10-20 is the only one with the 77mm diameter.
Tested on my Canon 35mm f/1.4 USM L, no problems at all, from min to max it was always fine. I went a bit extreme and I mount it on my Canon 135mm f/2 USM L, same story, no problems.
So last weekend I decided to meet my friend, and go out to take some test shots, take several shots with the same scene but with different settings of the filter, and also to see if maybe with another copy of the filter I get different results.
So down here you can see the shots I took, all of them taken at 12mm, f/20 and ISO 160, with my Canon 7D on a steady Manfrotto Tripod, obviously using a wireless remote to avoid any contact with the camera and get the sharpest result possible.
The first shot is a reference one, with no filter.

1/40 sec

The next one is with the ND Filter set at the minimum mark, so in theory at ND4, which is -2 stops of lights.

1/20 of a second

As you can see all fine here, apart I had only one stop of reduction of light, but the sun came out for a split second so that could be the cause of a different exposure, but let see what we get at half power.

1/6 of a second

So here we are gaining something close to another two stops, and the picture is still clear, so far it's quite good, I was expecting more light reduction at half power, in theory a 4 stops light reduction, but 3 stops are still good. The next one is with the filter set at 3/4.

0,3 seconds

Another stop down, from 1/6 to 0,3 seconds, not bad, but here we start to see that the clouds on the top left corner are darker than the previous shoot, same problem on the opposite corner, bottom right, the X vignetting is coming out, but the shoot is still fairly usable, even if in more extreme light conditions it would not, especially if you are shooting with a clear blue sky, and in that case the effect will be way more prominent.
But let set the filter to the max and see what happen.

4,0 seconds

This time we get 3 2/3 stops of light reduction, which brings the total to 7 and 2/3 of noise reduction, pretty close to what the company claims, and as you can see that the sea is starting to create the smoke effect as the exposure is getting longer, but at this point the photo is absolutely useless, maybe you can try to recover it with photoshop, but honestly I can't and I wouldn't even try, as it will result in a big waste of time only to correct a problem of the filter.

What about the other copy of the filter?
As I mentioned above, I tried to do the same test with his filter, repeating the same shots I took with my copy. I'm not going to show you the pictures as the results are pretty the same, his copy gave me slightly less problems, but at the same time it gave me a stop less light reduction, somewhere around 6 and 2/3 at maximum power.

But what if I'm not going to use it on ultra wide lenses?
Now, as we said before, there are no such problems using this filter with lenses with a bigger focal length, no problems at 35mm and I can say probably no problems with even wider aperture, like 24 or 28mm (even if I didn't actually test it with those lengths), but my friend uses his filter with his Sigma 17-50mm f/2.8 EX DC OS HSM, and it has no problems as well, even at 17mm, so I can probably think that this is only a problem with ultra wide lenses, like the Canon EF-S 10-22mm or the famous Tokina 11-16mm f/2.8.
You can might think "well that's fine, no?!". Well no, it's not fine, a filter should work on any lenses you have. Also, an ND Filter it's mostly used for these three cases:

• For long exposure photography
• For videographers who need to shot with daylight and have a better control of exposure
• For who wants to have a shallow depth of field, using fast lenses, under the sunlights

The last two points will not be effected by this filter problems, for sure not the third one, maybe the second one only if they use an ultra wide lens.
But my case is the first, long exposure photography, and to do so I need to use ultra wide lenses, not only because it gives me a wide angle of view, but also because it gives less problems in terms of sharpness, as the longer is the focal length, the easier you get blurry photos.

Image Quality
Any filter you put in front of your lens it will going to affect the overall quality of your lens, so if you use it with a not very sharp lens, don't expect any improvement in the sharpness of your photos, but if you have a good glass, this filter doesn't really affect too much the sharpness, it also add a bit of noise, but it's nothing that cannot be corrected in post production.

Build Quality
In terms of build quality, I cannot say anything really bad about this filter, it's not super refined, but overall is well build and it comes with a handy 82mm lens cap, as the filter on the outer side has an 82mm filter thread.

Conclusion
The Light Craft Workshop Fader ND Mk II is a filter that can be good or bad in base of what you generally photograph. If you use it with not very wide lenses, or for doing videos, it's a good product. There are better filters out there in terms of image quality, but I doubt you can find them for less than 100 euro.
But if you use it for landscape photography, especially for long exposures, with ultra wide lenses, like I do, then is just a waste of money, and you better save some extra money and get what you really need, the Lee Big Stopper.