Let me start off by saying that this plugin is perfect for what we need and I appreciate your hard work on it. There are just 2 things that would make it even better....
1. Admin (or even better, custom) email notifications. I'd even be happy with the "From" email being carbon copied when the user is notified that they signed up for an event.
2. Repeating events. Every Sunday, once a month, etc.
Other than those two things I don't see anything wrong with the plugin, and I can't think of anything else I would want to add to it. It's very simple, easy to use, and appears to be well-built.
Thanks for your hard work and making this available for free.