When we mailed out our invitations we made sure to include our wedding wire website link so people could just RSVP there as we have friends and family that travel or just never mail things. We have more people using the site than mailing which is perfect as we find out faster and don't need to worry about buying extra stamps (they're an added expense). We shared our website link in a conversation with our guests on Facebook as an extra way to keep contacted and for them to all get to know each other.
I’m using it! We sent out our invites last month and people have already started Rsvping. My FMIL changed the domain to make it more simple for the invite. The only thing that’s tough is if you don’t know the persons guests name you won’t have them on your guest list so they can’t RSVP. You have to add them on so we just made a note on the RSVP section of the website to contact us if your guest or child’s name is not found and we can add them for you.