I'm using SquareSpace. I'm not too fond of the free templates that WeddingWire and TheKnot provide. In addition, my RSVP is online.. so with the guaranteed traffic, I had to ensure the website is as beautiful as the wedding invitations.
Downside is it's $10/month (another wedding expense, go figure), however, I started putting it together 4 months prior to my wedding date (with wedding invitations going out 3 months prior to the wedding date) so I only pay $40 during the time people will actually use it. It also eliminated the cost of an RSVP card and postage.