I'm basing my opinion on more than just the trailer. I'm basing it on the fact that Paul Anderson is, at best, a C-grade writer/director. I'm basing it on the fact that nothing I've seen so far has impressed me. For the record, I thought the trailer was pretty average, but that's probably because I watched it without sound at work. It's easy to cut together a short trailer with quick shots of an already cool looking creature, but that has nothing to do with how good the movie will be. It takes more than the image of an Alien drooling, or a Predator shaking its spear at something to get the fanboy in me excited.
Hey, I love fun movies too. But I also remember people thinking that Van Helsing would be "fun."