My quick review (based on an hour of messing with it):
If you’re already writing in Fountain, Slugline is a very nice option. For a 1.0, it seems well-tested and quite thoroughly thought out. You can do all the Fountain-y things you want, including the Markdown-like stuff like making words italic etc.
Dropbox integration is easy, though there’s a permissions bug that will tell you that you don’t have permission to autosave to a dropbox folder, which is apparently getting fixed.
Saves files as .fountain, which can be easily imported into Scrivener.
Cursory observation about import into Scrivener: appends imported documents with a red note at the bottom that looks like this:
Is that an endnote? I don’t know about such things.
The trick of the thing is that, when you type in Fountain, Slugline “pops” each element into screenplay format. The “popping” is fast and responsive. Here’s a qualm: If you spend your day looking at the screen, is the constant shifting of text from the left margin to the appropriate screenplay margin going to get annoying? I don’t know. **
When you open the preferences menu, it says “I prefer to write” and has an AGREE button to dismiss the box. And that’s it. Which would be cute if it weren’t bullshit — because while there are no actual preferences under Preference, there is a Document Settings menu under File which gives you a few, yes, preferences. All of which results in the preferences simply being in the wrong place, all for the sake of a joke that doesn’t quite land. Fucking screenwriters. 
Autocomplete works well.
There’s an outlining bar on the left (optional), which does outlining in the exact wrong way if you ask me, but it does its own thing really beautifully, and if I learned it, I’m sure I would love it. coughFargo is awesome*cough.
You’re right, Mats, Slugline is expensive. It’s only $10 less than Scrivener! And it doesn’t export as anything but .fountain, so if you want to save your script as something else (other than a PDF, I guess), you’ll need to re-open your document in something that reads Fountain and can export as, say, .FDX (like Scrivener). Slugline does give you the option of sending your document directly to Highland for conversion. Thing is, Highland is already $20, so now you’re at $60 for the suite.
It sounds like I’m down on Slugline. I’m not. It’s actually quite wonderful, and it really does feel like “the new way to do things” in the screenplay app space.
If it’s a choice between Slugline + Highland vs. Final Draft, it’s a no-brainer. Even at $60, you’re WAY ahead on cost (let your producer spring for FD.)
But Slugline + Highland vs. Scrivener? Hmm. Scrivener’s screenplay mode and export options do everything the other two do, albeit the other way around. Plus, at the end of the day, you have Scrivener and all the other things it can do. So it comes down to how you want to write. Which is great — options are cool. I can see myself writing scripts in Slugline, and I’m sure it will be a great experience. But I collect these apps like barnacles, and since I’ve already sprung for it, I’m now looking at it as a sunk cost. Which is probably the wrong business school term, but whatevs.
I think my reaction to Slugline is the same as my reaction to Ulysses III: “This is AMAZING! Gosh, this guy did a great job! Why do I need it again?”
[size=85]** Theres a part of me that thinks the real “just write” app in this space is Highland by itself, which does much of what Slugline does without the “popping” thing. And I haven’t even mentioned Fade In, which is a hell of an app![/size]
) want to incorporate Slugline in their workflow, it should be noted that Scrivener imports and correctly formats Slugline files, and Slugline accepts Scrivener script files exported to Fountain.