Edit: I've read a few more comments and some a free with you, while others think it works fine. I'm wondering if there's something about the UI that clicks with some but not others.
Also, there's definitely something "off" when it comes to using it intuitively. I have to devote a lot of brain power to searching for functionality AND trying to comprehend exactly how those pieces of functionality are going to affect my library data.
Maybe it's too powerful of a tool up front or something. It could benefit from a "basic view" maybe?
None of these are complaints from me, I'm a happy user. I'll just echo some other commenters in wishing for a more robust network interface (whether that's just an API, or a more hashed out front end). I'd like to easily share my library with people or just other devices while on-the-go, and Calibre has all the device formatting functionality built-in.