Cheap 8 stop ND filter - any good?

[Rooster]

7dayshop (no, I don't work for them http://www.7dayshop.com/catalog/def...le+ND&x=0&y=0&filterwordsalldb=1&r=20110331-2) are selling a knock-off of the Lightcraft Fader ND filter which goes up to 8 stops. It's pretty cheap - 77mm is £33.99. Are these any good? I seem to remember that they work like a CPL and if the sun is in front or behind then they don't have any effect, don't know if that's true. And it does warn of vignetting on full-frame wide angle lenses, the filter looks pretty chunky. Still, there's a money-back guarantee if you're not happy with it and my B&W ten-stopper could go on ebay for pretty much what I paid for it. Is it worth going for this?

 

[wontolla]

Already several threads asking the same question!
Some have sent for one and will post thoughts when tried out.