It is, when considering the value proposition over e.g. RTM is the collaboration. $99 isn't terrible but by itself it's really got nothing over the multitude of other GTD options. To use it to collaborate, for just me and the wife, it'd be $198/year, if I understand the pricing correctly.

I'd certainly pay the $99 for a family licence which could be, say, 2 install-everywhere keys plus one just for desktop (for the kids).

RTM doesn't offer any of the collaborative features that Flow does, nor do they have a native OS X app. I use RTM, but mostly as a simple TODO-list that is geo-enabled.