Boltgun.
I've played a few Warhammer games, but this one actually does the weaponry justice. Even the starting boltgun is fun to shoot and turns basic enemies into chunky gibs, it's wonderful. The heavy bolter is one of the greatest FPS machine guns I've used, since it takes up a huge chunk of the screen and feels utterly monstrous to fire, even the bigger enemies are splattered everywhere by the time you let go of the trigger.
It even has a dedicated taunt button so you can scream at the heretics and there's a large number of lines for some reason so your Ultramarine rarely repeats them.
The downside is that the maps are just plain ass. You get no map, only waypoints if you press the map key, and some of the level design is outright terrible. The boss fights are shitty too, you get these special grenades in secret areas you can only hold one of and you better keep them for the Chaos Sorcerer at the end of each chapter because that broken shithead is going to kick your ass on the harder modes otherwise. What's strange is the game is super easy outside of that thanks to how beefy all the guns are.
In terms of 40k canon it's also fucking ridiculous. One named Ultramarine in this game kills an entire army of Chaos demons, a large portion of the Black Legion, about SIX LORDS OF CHANGE, three or four Great Unclean Ones and the powerful Chaos Sorcerer summoning/controlling them all. I like to think this is an adaption of his completely exaggerated end of mission report which the chapter master read for five minutes before tossing it aside in exasperation.Either that or Malum Caedo is secretly the Emperor himself.
So yeah, good game, annoying flaws.