Answering a couple related questions... I think that for all practical purposes (photographic ones at least) the strength of the effect should be essentially the same using a circular polarizer or a pure linear polarizer. My guess is that cases where people have reported a weaker effect with a circular polarizer may be just due to the person looking through the filter the wrong way. More generally, I would say that their characteristics are likely to be mostly very similar (outside of the whole issue of needing a circular one for some cameras), and I can't think of any reason to worry that the circular polarizer might be any less effective in use than a linear one.
Having said that, it is certainly true that there are quality differences among different brands, there can be some sample variation within a brand, and perhaps even an occasional bad egg that was manufactured or labeled incorrectly. But there are so many other considerations that probably rank higher on the list of what to look for in choosing a polarizing filter - things like AR-coating quality (less expensive ones may be un-coated), maximum transmission, color uniformity (ideally NO color cast), optical wavefront quality, scattering or flare tendency, uniformity, durability, etc. Usually you will be safe with any of the top brands.