I just watched this via Netflix stream on Friday, and was riveted for the whole 2.5 hr running time.
I'm not that into Tilda Swinton as Great Actress, so all the praise that I heard heaped on her performance in this made me suspicious about whether or not I'd like it. She's great in it (small-'g' great, but that's another story), but just in terms of the movie as a thriller - WOW. It's one of those great rare cases where it establishes early on that the main character is capable of anything, and that the movie will not turn away but will follow her all the way down the rabbit holes of her crazy choices, so the rest of the movie has that "anything can happen at any moment" tension that makes for a great, twisty thriller. If you come into it with those expectations, rather than looking at it like a MONSTER-esque character study, you'll get a great movie in return.
It reminded me of seeing NO COUNTRY FOR OLD MEN for the first time. Every time Javier Bardem was on screen I had that same feeling ("Is he going to politely buy some gas and leave, or is he going to slaughter everyone in sight?"), and every time he wasn't on screen I was worried he was going to jump out from somewhere in the background at any moment. Same thing with JULIA. For the whole movie - especially whenever she starts waving that gun around - I was sitting there with my stomach in knots wondering "what the fuck is this crazy chick going to do next?" Great fun at the movies.