It's the little things that kill, but in evite's case, there is a dinosaur-sized flaw in their user experience.
It is, quite simply, that evite allows you to create an invitation before signing up or registering. HOWEVER, if you do create the invition before being logged in, the site will prompt you to login or register to complete the invitation. And, once you login or register, your invitation is completely deleted and you have to start over from scratch.
WHAT THE F********!
That's generally the reaction of people who experience this... or, at least that was my reaction.
There are many other user-experience issues with evite - apparently, so many that there is actually a website dedicated to finding alternatives to evite: