Be very careful you read all the conditions attached to the reservation.
The most significant.
Reserving seats costs money and must be done prior to check-in. If you want your party to sit together, you will probably want to do this.
Checking bags costs money and costs significantly more if you pay at the airport. Book your checked bags in advance.
You must check-in before going to the airport. You can do so from 30 days before the flights, so no need to wait until 24 hours like most airlines.
Make sure your single piece of carry-on meets their size requirements. If it's as much as a millimetre over the limit be prepared for it to be taken from you and significant fees to be taken.
An LCC makes its money from selling extras, particularly from people who haven't read the rules properly.
You will have to uncheck the "travel insurance" they try and impose on you multiple times during the booking process.
Finally, booking car-rental can be (but isn't always) a good deal through Easyjet.