I've seen Terminator, Abyss, and True Lies and
parts of Aliens and Terminator 2. And his dialogue isn't anything to write home about. It gets the job done, but it doesn't move me.
It's definitely not the worst thing one could say about a film - if someone's main complaint about a movie is dialogue, then it's actually a good sign as to the overall quality. Bad movies have MUCH more to complain about that just bad dialogue.
Cameron's a good storyteller. You know what's happening in his movies, you know why they're happening, you know what his characters' motivations are, and the sequences are emotionally compelling. Dialogue is pretty low on the list when it comes to actual storytelling, just as continuity is low on the list when it comes to editing.