Filters - worth spending the money?

[glypo]

Well I'm enjoying my new 40D and with Farnborough Airshow coming up I've been looking at filters.

I really need a circular polariser, partly under advice before so I can get darker skies, but also to stop nasty reflections. The problem is, there are so many makes of filters. Is there any major difference between the different brands.

They start at £22 with this Hama upto £125 for this B+W. And anything in-between (Hoya seem popular).

Google has been little help, so once again I would like some wisdom from the great oOple forum members. I spent nearly £900 on my camera, so don't want to ruin optics with a cheap filter... but will It ruin the results or will the difference be negligible? Thanks in advance for any help!