First of all, I love a good creature feature and a low-budget B horror movie.
These movies are basically what Night Shyamalan calls "popcorn" films- made for you to have fun with. They also have "heart," and seem to bring us a lesson.
I agree that there were times that it was slow, but the acting and the scares made up for it. Also, too much time was spent on the grief between the husband and wife, but it wasn't enough for me to dislike it. Also, with low-budget films, it is usually best to show the creature slowly. I loved the way that it was revealed in small, creepy slices. The CGI was brief, thank God, but overall, I loved how the monster was revealed.
(Spoilers ahead!)
That being said, the last few minutes killed what the director had set up. In my humble opinion, I believe we could have learned that there were more of these creatures (such as cutting to a shot of them in the woods) without feeling like all of the sheriff's efforts were in vain. This should have had a sequel if they were going to end it like this. I mean, the director and the script spent so much time building up the empathy for the characters, that it was a waste to realize that they were ultimately doomed. Overall, it was a great monster movie!