I remember when The Terminator came out. I was in college, and I hadn't heard of it. A friend told me he heard the movie rocked. So we went to see it, and after the first 15 minutes or so I was not convinced. But it kept getting better, and finally, by the end, I was a believer. The movie was awesome. And the time travel was internally consistent! Wow.
I saw T2 in grad school, and I couldn't believe a sequel could be that much better than the original movie, which was already pretty excellent.
Then I saw T3 and realized that sometimes sequels suck. In fact, the story line wasn't bad, but the production values weren't terribly high, and the acting/directing made it feel like it was made for TV. The sweep of the first two movies was gone.
So now they're making T4. I think it continues on from T3, which would be too bad, but maybe they can make it work. The trailer is... interesting. Not sure what to make of it, but I guess soon enough we'll find out.