Easy to install and use.
I installed the latest version of Calibre after reading all of the great reviews, and I've had nothing but trouble. It runs slow. Painfully, insufferably and uselessly slow. In fact, I can't do much past loading the main screen before it completely locks up my system. Well after ~5 hours, 2 re-installations (same and older, 'more stable' version...0.7.17), 5 forced restarts, 1 windows failing to start (never seen before)...I give up. I've tried pretty much everything to try to get this working. (includes disabling firewalls & antivirus, and every other 'fix' there has been to this problem) I'm running windows 7 on a computer more than capable of handling whatever program I throw at it - except this one apparently. Every time a user (like myself) reports this problem, the developer swiftly dismisses it as a user-end problem...so if this happens to you, instead of spending hours of messing around just uninstall and move on.
There are no 4 star reviews.