For that price, you get the normal stuff you'd expect and also:
- no ads
- ability to set each invitee as an adult, a child or an infant (useful when planning food and activities)
- ability to group invitees into families, so that any adult in the family can edit the RSVP for any/all family members
- single place to manage all messages with attendees
- easy to start this year's invitation as a copy of last year's
- app that displays the total 'yes' folks, split by adult, child, infant
It's totally worth the money for the time it saves and the ease of use for guests.
But I'm surprised that AFAIK there's no open source thing that's as easy to use, and that includes all the features I listed.