It was nice to be a member of TheKnot for the wedding planning tools and all the free offers and deals (we got our wedding guest book from Shutterfly for free through TheKnot). The planning checklist helped me initially as I got started with the planning process, but then I never used it again after I made my own checklist.
My complaint is that the wedding website builder was very frustrating to use. It was SUPER slow most of the time, and about half the times I logged on wouldn't even load the page I was trying to view or edit (This was not an issue with my internet connection. Every other webpage I would use during the same browsing session would function just fine). It was convenient for many of our guests to RSVP online and for me to get email alerts when they did, but for several guests the online RSVP didn't actually work. I was so glad that I created a Google spreadsheet where I recorded all the RSVP information from email and snail mail, because I frequently could not get into TheKnot's guest list manager to view RSVPs.
If you're considering using TheKnot for your wedding planning, go ahead and create an account for the details and planning help, but build a GoogleSite or other simple web page for your wedding website. It will save you a lot of time and frustration.