Mostly, they are bad. Usually it's killed after the second movie. And then there are movies where the producers didn't know when to stop. It sickens me to think about them. Brrrgh, I'm getting shivers all over.
Trilogies can be a good thing.
And you all better be there. But honestly, you can sit down to a good, quality set of movies. Trilogies are good that way. Get a bunch of friends together, rot your brains for six hours and eat lots of junk food. Go for it.