I said it before in another thread, but I'll say it again here: filters aren't a case where you get what you pay for. I have a bunch of cheap $15 Hoya UV filters for almost all of my lenses, but when I bought the 12-60mm I bought into the argument that it's a waste to buy an expensive lens and then put cheap glass in front of it (which hschnee
mentioned as well).
So for the 12-60mm I put on a Hoya HMC Super UV filter, which cost around $80. To be perfectly honest, I feel like a fool for having bought that over one of the cheap filters. I don't see any
difference when compared to my cheaper filters. The Zuiko Digital 50mm macro, which has a cheap $15 Hoya UV filter, still out-resolves the 12-60mm. Some people claim to be able to see a difference, yet I've never had anyone tell me "that picture would have been better if you'd just removed that UV filter."
I've seen someone do a side-by-side comparison with a filter on and off, and if that test of his is to be believed, then there is a bit of light loss with a filter. In theory a better filter should result in less light lost and better flaring protection. Can't say I've had issues with either of those even with the cheap filters, though...
Finally, there's someone whose opinion I trust and respect. He had two circular polarizers; one was around $18 and the other was $80, if I remember right. He said that there was barely any difference between the two, and that the $18 one was actually slightly better.
So there you have it. If you have money to burn and/or you'll feel better about your photography by having a very expensive filter in front of your lens, then you know what's right for you. If you're just buying a filter for some peace of mind about protecting the front element, then go with a reputable name (Hoya is my preferred) and get their cheapest one.
Final note - I'm not saying that there's absolutely no difference between the more expensive filters and the cheaper ones, because I'm sure there is. I also don't think that anyone who buys an expensive filter is foolish - only me
I just don't think that the difference justifies the price difference, especially for a hobbyist.