I’ve never been happy with the state of event planning. Sure, the most people are on Facebook, but not everyone is, and of course, it’s Facebook. Most other services are hit and miss, Evite is terrible, and good ol’ calendar apps are always in various states of not working with each other.
Savvy is an interesting entry. App seems good so far, it has a nice simple flow, and it has the option of allowing your participants to decide on a time. Most importantly: people don’t have to sign up to participate. Give it a shot.