Nearly all the pictures that include skies included the use of this filter.
http://www.flickr.com/photos/chrishamlet/sets/72157626177132496/with/5490368516/
There is one at the end where I went completely bonkers with the colour of the sky.
I would like to point out that I have no problems paying for expensive kit if it's going to be used regularly, however for experimenting or occasional use then I wont pay £70 for something that stays in my bag.
As to testing it against other filters then I'd have to buy those filters, kind of defeats the object.
I've used my 77mm CPL ( price a Hoya one but sit down first ) on numerous occasions over the last few years.
What can I tell you, well with or without the filter I can't see any difference in sharpness. I don't get any wierd colour distortion, no noticable CA. No flares, and it's thin enough not to cause Vignetting.
If anyone would like me to try anything specific I'll try my best to test it asap.