Except when you have pdfs that use certain funny image-compression things that make it look fantastic in Acrobat and xpdf-based viewers; Preview does horribly on those. I agree in general about Preview, though. (It might be jpeg2000; I just know that when my dad scans a book, it looks good in acroread and xpdf, but in Preview it's very slow and pixelated.)

Preview is good but I like skim a bit more. It has the note taking features and smooth zooming with trackpad pinch gestures. I cannot believe how fast it is too! Great app. One thing I wish it had though is the pdf editing commands that preview had to remove a page and concatenate different pdfs together.

Also: foxit reader is a great reader for window. Not quite as polished as skim but still the best reader I have come into contact with on the windows side of the fence.

I guess you've never tried PDF-XChange or Sumatra PDF, then.