I really like many of their UX ideas, and the fact that they use rtf and files instead of a proprietary binary blob.
The big problem I have with scrivener is that it seems custom-built to prevent sharing. Just read the horror stories on using dropbox... you need to close it on one computer to start working on another. And getting comments from another person not using scrivener means a roundtrip to word format or similar.
Writing outliner, their main competitor, has the 'proprietary binary blob' problem (an sqlite db, but still, not files that you can share without exporting first). However the author is going through a full rewrite that will make it use 'normal' files, which would make it preferable for me.
That's not a problem with scrivener, that's a problem with the way that dropbox handles files. There was previously something to do with the resource fork on the mac and that causing issues as dropbox didn't sync that, but that's fixed as of dropbox 0.8. There's another problem in that scrivener has the files open all the time, and dropbox does some pretty wonky things with those, like renaming files to myfilename-currentcomputername.ext. Other apps that keep files open have the same problem- try using a sync program with a live visual studio project, and you'll see the fireworks start. Use scrivener locally, sync that to dropbox, then use it locally somewhere else works. Some apps just aren't compatible with having multiple copies editing the same file at the same time because they keep a lock. That's why word has that sharing setting that's been getting better over the years.
There's also the alternative that they put in to address this- using simplenote. Simplenote is a free service, and the syncing with Scrivener works. The reason this works is that the syncing is controlled. I'm not sure if multiple people have the same note open how that particular problem is resolved. But for syncing your own project across multiple computers, it's gold. And I hope that they put it in the windows version soon!
I also have and use writing outliner, and I think that even once he does a rewrite to get rid of the proprietary slob problem, you'll see the same problem. It's the workflow, not the app.