I can't give this product any kind of fair review as I can't quite figure out what it's for. If I had to guess, I'd say this -- and the similar Scrivener -- are for writers who may indeed have the prose chops to get the job done but can't get a handle on how to organize longer manuscripts in their heads, and they also like to keep absolutely everything on their Macs.
The Timeline feature I thought would be the most useful -- dates are something I do sometimes flub -- as I expected I'd punch in a scene, give it a date, it would show up on the timeline. Apparently, however, you have to associate scenes with events, and the timeline is then TOO detailed: I don't need to see hours, let alone minutes, but rather just an overview so that I can at a glance see where I probably haven't misdated a significant event having cascading effect on future events, or to which preceding events ultimately arrive. Sometimes, not infrequently, I need to see a span of DECADES all on one line, all at one time, and this seems impossible with the software.
Foremost, StoryMill and Scrivener are not models for how novelists I know actually work. We have various loose "processes", we keep notes, we do research -- not too little research on-the-fly, so to speak -- and we may rough out in a notebook, on an index card, or on the back of the power bill, overarching plot lines, concepts, perhaps brief character sketches, snippets of especially pithy dialogue or metaphor we just have to use, that sort of thing. But everyone I know merely takes something that's been stewing, sits down one morning, or evening, or dead in the middle of night, and begins writing; then we go back and eradicate, illuminate and, well, prevaricate, as required to make the story whole.
I can see how software of this type might be beneficial to those writers -- serial television comes immediately to mind -- who are regular employees, who absolutely Must produce a story, and MUST produce it on a deadline, often using many of the same characters, plot histories, locales and preceding events in developing their plots.
As for aspiring novelists, screenwriters, playwrights, etc., I can't help but advise you'd be far better off just sitting down and writing, ignoring the confusing disorganized mess you may create -- because you CAN -- and WILL, if you stick at it -- develop a process for sorting things out, making sense of disparate parts and gluing them together into a coherent story. The bottom line is, called upon to take 60,000 - 120,000 words or more, vet this draft for grammar, style, continuity errors, etc., it's never going to just wrap up nicely, and it's always going to degrade into a brutal grind at times, whether you write on legal pads with a blunt pencil or with these sorts of computerized writers toolkits. It's never easy, no matter what.