I've used Flow for the last few years and it's served me well for large team projects as well as quick projects. I'm also close with the team behind the product and have thrown far too many suggestions, bugs, and referrals their way ;)
For me, the primary draw to Flow is that, before anything else, it's personal. By that I mean, its geared towards the solo user, but still allows you to tailor your workflow for collaboration. Many other products (Basecamp, ActiveCollab, Asana, etc...) are more feature-rich, but from personal experience the feature overload results in individuals lose focus, doing things differently, and slow-to-no-adoption due to the learning-curve. While at Grooveshark, we worked with probably every PM tool imagineable. Observing the behavior of individuals, it was obvious that more wasn't better, but neither was too-little. The ideal product was one that allowed each individual to tailor their workflow to their liking, but that still encourage discussion and awareness.
I do agree there are still some things missing from Flow, many of which I know they are working on in their next release scheduled for the Spring, but when you compare it to other alternatives, it's a definite winner.