There is a post on here at least once a week of someone complaining about TK RSVP. I wouldn't use it. We are using weddingwire.com for our website and RSVPs and haven't had any problems. I'm backing everything up on an Excel spreadsheet as well.
Use postcards. You can get them vistaprint very cheap... and they are much nicer than an electronic RSVP and you won't have all of the hassle everyone does with e-rsvps.
Websites/blogs where our wedding has been featured:
Re: TK RSVP
Websites/blogs where our wedding has been featured:
http://www.dapperq.com/2013/11/a-very-dapper-wedding/
http://www.onabicyclebuiltfortwo.com/2013/10/wedding-christina-g.html
http://4realequalityweddings.com/2014/05/16/g-christina/