I've been using AI1EC for a number of years now and I have to admit that it beats the (you know what) out of everything else I tried before finding this plugin. Given the fact that there are an almost unlimited combination of OS's, templates and other plugins (not to mention user skill levels), this plugin is just about bulletproof!!
I moved to AI1EC when the old calendar plugin I was using choked on the WP 3.5.1 upgrade and the author was too busy to work on a fix. AI1EC is packed with outstanding features, is flexible and easy to use. I've also found the staff to be extremely helpful.
In my mind, you can't ask for more.