I originally installed the free version, but upgraded to the paid version, and it has been well worth it. I was able to synchronize with multiple Google Calendars, so that reservations created in PB Booking appear in the Google Calendar, and vice versa. I also created multiple custom fields, integrated PayPal for rental fees, and changed the front-end styling quite easily.
Eric, the developer, has been SUPER helpful in responding to my many questions. He has fixed bugs, integrated feature requests, and even corrected me patiently when I reported bugs that turned out to be just mistakes on my part.
Read the feature lists carefully to be sure you're getting the correct version for your needs - but don't hesitate for one minute to purchase the paid version. This is a great extension with even better support!