I've been using skim for quite some time now and I'm perfectly happy with it. It replaces preview for me completely. Here are some suggestions, though, which I personally would like to see:
1. it would be nice to have some preference settings which opens all documents with some default zoom
2. Something like the Safari history menu item would be great (with items 'earlier today','yesterday' etc) - probably very helpful for those who work with lots of pdfs every day
3. When I'm working with many documents opened at the same time, it would be nice to have tabs, and some hotkeys to jump between tabs/windows

Please continue the excellent work!