Surprised to see so much hate for Calibre's UI here. I'm not sure I understand. I can load damn near anything that pretends to be an ebook into it, and then I can double-click on the row for that item, and it pops open a reader window. That's all I need it to do.
At least on Windows, I've been pretty underwhelmed with other ebook viewer apps I have tried. The various Kindle apps tend to blow for anything you don't obtain through Amazon, and often don't work great for syncing even Kindle ebooks. I still have a bunch of Microsoft Reader .lit files kicking around, and Calibre is one of the only things that will open them. The Calibre PDF reader is pretty terrible, but setting is up to pop open to Adobe or whatever instead doesn't take much ceremony.