When I had a Mac, I hated the Calibre reader for its slowness, and honestly, it was unnecessary because iBooks is pretty great regarding typography and formatting.

However, on Windows, I've found that the Calibre reader is the best. I've found that Adobe Digital Editions and the Kindle reader have terrible formatting on a non-mobile screen.

I use it on my Mac all the time. I've never had it crash on me. The reader...is not so good.

Some faulty pdf\epub files maybe, but none of them crash any of my other readers, so... I gave up at some point. I love Calibre as a project, but for now I can't really say the same for the product.