we use this regularly at duxter and it's by far been the best way we've found to minimize the cognitive barrier that exists between design specs and a functioning site.
It's a powerful tool for creating interactive prototypes and managing the design process. Easy to use, it provides convenient collaborative editing and project discussion. An excellent choice for web designers and development teams
I've been a user of Invision for around a year now and never looked back, recent months has seen me throw in some Watch UI in their Web View but this is going to be way better! All I have to say is Invision helps me every day look even more professional than I probably should look. ;)