I totally second the above emotion, and absolutely cannot stress enough how great the original films are and that you really shouldn't let Prometheus be your barometer for the franchise. The original Alien is a bit slow but worth it, and Aliens is the exact opposite being a nearly exhausting actioner, but also worth it. You're also gonna need to set aside some recovery time from both!

The other two films Alien 3, and Alien Resurrection are interesting films, and while they're not awful, they are not as good as the first two, but still better than Prometheus.