I'm really curious… what are the unique features for (Snow) Leopard?

Top comparison, not available here:
Preview, "Mac OS X Viewer for PDF, Postscript, and image files".

I've been using Postview for a couple of weeks (get a temporary key from the author to get rid of the big DEMO text). This is a pretty good PDF viewer, much more functional then Apple's barebones Preview. It defaults to making pages fit the screen and can display 2-up as well. Postview will display the outline for PDF's that have been created with one, which lot's of technical documentation has. The search feature alone is fast and _extremely_ useful and I like how it highlights the line in yellow. It also displays additional information on the document, such as fonts used, creation date, etc. The only things I wish it did (which the author is working on) is to copy the PDF text to clipboard as formatted RTF (his TextLightning program does this) and that it had the option to display thumbnails in the document outline). It also handles PDF's that have landscape/portrait format pages, which I have had problems with in Preview. Postview can also render plain postscript files. I certainly haven't experienced any stability issues and have made it my default PDF viewer as well. Adobe Acrobat might do some of these things, but takes forever to launch and isn't a cocoa native application, which Postview is. I was skeptical at first, but quickly found that I use PostView daily now and have registered. If you need to read lot's of documentation and other PDF's on a regular basis it's well worth the price.