I think some can be decent, but far too many more seem to be just awful. I remember being so disappointed by the Super Mario Bros. movie, and I was quite young when I seen it too (in theaters, no less).

I will say that I have more good experiences with video game movies than bad. I did enjoy the first Resident Evil movie quite a bit, and the first Mortal Kombat movie was pretty enjoyable too. I fan-gasmed over Advent Children and the Silent Hill movie was pretty good. None of them make it in my favorite movies list though.