Doom3 can be a fun game, some chapters are better than others (alpha labs and hell aren't among my favourite ones). Thing is, the whole "imp spawns behind you" and "jumps at you when you open a door" gets old quickly.
Chainsaw is great, though. Sawing through one of those tentacle goons felt very satisfying. Well, and then there are those chaingun wielding bastards...
(wish the chainsaw would've been more effective on imps, or I did it wrong...)
Plenty of things to find as well. Take some time exploring the levels, you never know what you might bump into. Lots of goodies, nods to the other doom games and some hidden messages...
Overall I've always liked playing doom 3, but the insane combat of doom/2 always prevails. You can't compare really.
The finale is just too easy.