I just saw it. I liked it and thought it was fine for what it was going for. The soundtrack was awesome but maybe a bit too on the nose, and I wasn't feeling the ambiguousness of the time frame because it didn't give the movie a timeless feel as much as it was just distracting me.
Also, I'm pretty sure that you can only pass it on once, and then it's just a matter of time before it comes back to you. So it's not like you could fukk 20 people and it would have to kill all those people before getting back to you. You pass it to one person, and either that person passes it on, or is killed before doing so, passing it back to you. I think this is made clear when Greg has sex with Jay and is then shown flirting with a girl, implying that he passed it on as quickly as he could (intentionally or not since he didn't really believe in it) and that's why it took so long for it to return to him.