Dark Sector is an amazing game, as far as I'm concerned. Loved every minute of it, beat it multiple times, paid full price when I bought it after beating it once as a rental, and I don't even feel slightly bitter that it started selling for peanuts a couple of months after release - it was more worthy of my $60 than most games I've played this gen.
The 'generic enemies, bland environments, no story' criticisms make perfect sense, but Dark Sector has so many high points that the few elements which were sub-par didn't bother me at all. The glaive alone makes the game worth buying, but with the boss fights, puzzles, RE4-style weapon upgrade system and crazy superpowers; I never ran out of things to love. Plus it looks great, and showcases some of the best sound design in gaming.
I have no idea why people hate it so much. I'm just sorry the game will probably never get a sequel.