We are a small non-profit organization, that want to lend-out a piece of equipment to our members. For that purpose the WP Booking Calendar is perfect. Untill now we have only used the free version, so that is the only version I can comment on.
It can be a bit tricky to setup, especially if you want the responce mails in your native language, but once you are there, it is very straight forward and easy to use.
One thing to note is when you change anything on the setup-page, don't forget to klick the "Save" button, which is located at the very bottom of the page.
If you should get stuck, there is a FAQ to answer the most common questions, otherwise the support people are very fast and competent to help you out, and that goes even for the free version.