This film really wasn't bad at all. It obviously had a limited budget, but compared to similar offerings, 'Grave Mistake' produces a surprisingly entertaining 90 minutes of viewing. There isn't anything particularly innovative here; it's a zombie movie that follows the usual pattern and uses the kinds of stock characters that can be found in many horror films. Indeed, the film's strength is that it understands the genre well and therefore fits neatly into the niche it's aiming for.
The special effects here are surprisingly good as well, considering the type of film. The zombies look a lot better than can be found in most low-budget attempts and the gore is more convincing. It's a long way from perfect, but it's not laughable, either.
I think Seth Darling did well in the lead role for someone who has no other credits to his name. Then again, I can't stand overly Hollywood young actors, so I might be biased :) The other cast members range from okay to awful, much as one expects from this kind of film.
All in all, though, there are worse ways to spend 90 minutes. One to avoid if you want your horror Hollywood perfect, but if you don't mind a little rawness, it might be worth a try.