It isn't ALL scripted. But you can tell which bits are and which aren't. I'd say it's probably 25% scripted.
You couldn't script most of it anyway, it would be logistically impossible - and criminal, in some cases (ie, Snooki getting punched at the bar by that dude was very much real as he got prosecuted for it, same with the guy that Ronnie fought with on the boardwalk in Season 1, that guy tried to sue Ronnie for assault).
They might say "Hey, you all need to go to the club tonight and we'll film there" for example, but everything that would then happen, actually happens.
You gotta question the whole concept of these shows from day one anyway... purely on the basis that there's cameramen there and people obviously act differently when they know they're being filmed. The reason Snooki is the most famous out of all of them is because she was playing it up for the cameras from day one. The very first episode when they all meet each other at the house - she gets completely wasted and goes to work late the next day because she was throwing up. That's not scripted in the sense that they told her to do it - but clearly she was aware of what she needed to do to garner the most attention from the viewers.
Season 1 is still the most honest though. Reason being, it didn't air for months after it was filmed and thus none of them were known, or famous, throughout the whole of it, or had any idea how it would be edited and how they would be made to look (personality wise) in the final edits for broadcast.
Season 2, they're famous, people treat them differently, if one of them hooks up at a club you can sort of tell that the person hooking up with them obviously knows who they are and what's going to happen, there is obviously more of an agenda because you get the famewhoring people who will hook up with the cast just to get their 15 seconds of fame on a TV show...... and of course, the cast themselves now know what the public perception of them us and thus they live up to the caricatured versions of themselves that have been created by MTV.
I very much enjoyed Season 1 and 2. But Season 3, I haven't been into as much. It's been finished for ages now and I still haven't watched the last 3 episodes or so. I'll get around to it though, but for the reasons above, it's just not AS good now, and when stuff does happen, there's this "outsider awareness" that you can detect from them which makes it all a bit weird. Hopefully the change of country will negate that stuff in Season 4. I'm pretty sure that it will.