First and foremost, Flow is a web app that many will find very similar to MobileMe in aesthetics. Through this interface, one can add and manage tasks through projects or even delegate tasks to other Flow users (a handy feature for teams). The real killer feature we're seeing with Flow, however, is the cloud-based architecture. Combined with Metalabs' new Flow app for iPhone and its Mac companion app, it's a pretty great way to keep things in sync.
Flow has a 14-day free trial, which I highly recommend trying out. Beyond this time frame, Flow has a monthly cost of US$9.99. Metalab is also offering a yearly subscription for the very MobileMe-esque price of $99.