I had one or two small issues going from 1.8.4 to 1.9.1 then 1.9.2 but, given the fact that there are an almost unlimited combination of OS's, templates and other plugins (not to mention user's skill levels), I can't complain, especially when I think about the money I have invested in this - zero!!
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 has some great 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.
Should be WP 3.5.1 - I forgot to change the version on my previous comments.