Personally, I make three distinctions with movies:
- Liked it/it was good
- Didn't like it/it was bad
- Couldn't finish it
I won't say I did or didn't like a movie unless I've watched the whole thing, regardless of how good or bad the first half hour/hour/whatever was.
For example, I've never finished Lost in Translation. I don't have much desire to, either. But I don't know if it's a bad movie or not, because I haven't seen the whole thing.