The first time I watched this movie, I though it was terrible. However, after watching a 2nd and 3rd time, I've learned to appreciate the humor. It's a fun and lighthearted movie. The special effects are quite good, the monsters are impressive and the acting is not bad. David Duchovny isn't a great commedian, and some of his lines are terrible, but overall, he does a good job.
It's the kind of movie you put on when you are depressed and just want to drink a beer and be amused. In the same league as Caddyshack and Stripes. Great mindless fun.