Flow is aimed particularly at creative teams and agencies.
That interests me because creative businesses have a strange relationship with project management software. They need structure badly, but they often hate feeling overmanaged.
Flow gives teams timelines, responsibilities, project visibility, and shared information without making the entire operation feel like an accounting department.
The drawback is the same as every project tool. Somebody still has to update it.
If your projects currently live in several email threads, a few chat channels, somebody's notebook, and a spreadsheet nobody can find, however, you probably need something.
Pros
Good project visibility. Workload management. Central place for project information.
Cons
Still another system employees have to maintain.
My take
Useful once a team has clearly outgrown email and memory as its project management system.