Linear polarizers are normally used with non-autofocus cameras. I assume that a circular polarizer would be required with the G1--I could be wrong though. All of my polarizers are circular as I do not want to have to worry about what type of filter I have on what camera. Circular works fine with ALL lenses/cameras but are more$.
As for brands. I have a bunch of Tiffen ($$), Hoya ($$) and a few B+W ($$$). I do not have any Heliopan ($$$$). As far as I am concerned all that I have are about the same. I would stick with one of the "big four" however as it doesn't make sense to put a cheepo off brand filter on a good/expensive lens.
I consider Sunpack ($), Lensmate ($), Bower($), etc. etc. off brands.
On the other hand I do not need the brass ring on the B+W for the extra $$$.
A word about Tiffen. Some self proclaimed "experts" bad mouth Tiffen. However, they have been around for a loooong time and have long been a leader in filter technology e.g. their 812 warming, enhancing, Hollywood FX series etc. I have had excellent results with Tiffen and I figure if the're good enough for the movie industry to put on their $100,000 Panavision cameras--they are good enough for poor ol' Dick.
See www.photonet.net
Regards,
Dick S, Temecula, CA