Seamless, super fast, easy to use, very intuitive interface and functionality, fully integrated into Google's ecosystem but also friendly with third party options. Easy to incorporate and Sync icloud's calendar and other options so you can consolidate all of your calendars in one place.
Easy to export as well for integration into third party calendars.
Available as a standalone app in cell phones and tablets, or just by instantly logging into a webpage, reminders and alerts available even for the webpage version.
Lacks a standalone windows app.
Could use better desktop integration, interface customization lacking but maybe that's just being picky.
Unless you're required to use icloud, Google calendar is the best fully featured easy calendar options.
Google Calendar integrates seamlessly into all google platforms and OSs, has allowed me to handle appointments and reminders and even import and export from and to icloud calendar for full compatibility.