What do you like best?
CalendarAnything is the most flexible and robust calendar tool on the AppExchange. It allows easy calendar visualization of just about any data in Salesforce, supports both standard and custom objects and is packed full of features like custom filters, conditional coloring, dynamic calendars, printing, sharing and exposing to the web. The UI is very intuitive and is similar to Google Calendars. It is very simple to admin as well.
With the latest release (Winter '13 at the time of this writing), the feature set has doubled over Spring '12. They're constantly adding new features based on the requests of their customers and prospects.
What do you dislike?
It carries a nominal fee, which means it's only worth it if you can justify spending money. Otherwise, other less powerful but free apps might do.
Recommendations to others considering the product
If you have a need to visualize Salesforce data as a calendar, for instance, marketing campaigns, case management, customer service, or sales lifecycle, CalendarAnything is worth a look. It's flexible enough to meet most use cases, whether within the sales or service cloud, or for a custom app. Installation and config is a breeze, and you'll be up and running in no time. Also, support is very prompt and helpful if you have questions.