I just watched it tonight... I didn't see the theatre release so I hadn't seen that ending, but I think I like the director's cut better... not so "happy sappy". The thing that got me is that it's a better film that I thought it might be (hoped it would be). Rarely am I ever particularly struck by a major release, so this is quite a compliment coming from me. In other words, see this film.
I am a sucker for any flick about time travel so I'm biased, but I thiought it was a great one. IMO, Ashton Kutcher should play serious roles more often cause he isn't half bad at them!
I liked the theater ending better but I also prefer appy endings cause the director's cut ending is just plain tragic!