I went to it with very very low expectations, and came out with a smile on my face. Thats worth the £8 fee in my book.
It doesnt really try to be too serious, and it has a few good links back to the Doom fans (Dr Carmac, Pinky, BFG, etc).
The fps part was fun and exciting. It was sort of like the alien view in Aliens. It was an interesting twist to get God Mode into the movie.
I think its either one of the best B films Ive seen, or a poor to mediocre A film. Ill probably buy in on DVD when it comes out for a good laugh every now and again.