I use it. I like it, but with caveats to follow.
I really like it as a writing tool. I never thought I'd give up Word, but once you get used to Scrivener (took me about a week), it's a lovely tool. I like being able to use the corkboard to put notes on a chapter. I like being able to easily insert placeholder chapters in the side-outline tool. I like being able to see the whole novel at a glance.
I like having side areas for character bios. I LOVE being able to highlight something I've just typed about Aimee's Mother's Favorite Pizza Topping and right-click and have an option there that says "Copy To --> Aimee's Parents Bio". Sooooo convenient for a writing tool.
The tool isn't perfect; it still feels rough around the edges when it comes to text formatting and paragraph options, but the development team seems willing to grow.
Now. Exporting. Trickier.
The Scrivener to Word export is clean enough that that is what I sent off to my editor (who works in Word). The Scrivener to ePUB export... is cleanish. I still had to spend several hours tweaking things in Sigil to make it perfect. BUT! That is MUCH LESS time than I would have had to spend if I'd been writing in Word. So my two-cents is that Scrivener --> ePUB --> Sigil --> ePUB is the best process for me.
As for the mobi, I would recommend cleaning things up in Sigil for the ePUB and then using Calibre to convert the ePUB to mobi. As long as you turn the automatic chapter breaks off (that gave me some trouble), the Calibre conversion is remarkably clean when the starting ePUB is spotless. In my experience.
