For my last party invite, I tried using facebook but a lot of people balked at it. Pretty sure the reason was that it forces you to think that you need to sign up/log in and people just want to have an easy access to event time/address, and a yes/no button to rsvp.

I just created a test on anyvite and it was super easy and common-sense. Definitely preferrable to facebook in my opinion. Two comments though:

1. RSVP dropdown is smallish/hard to notice. 2. Background images stretch in an ugly way, at least on my work IE 1024 x 768.

Otherwise, a great site!