Picked up Warhammer 40,000 Inquisitor - Martyr on the Steam Halloween sale last week. Reminds me of Diablo play style set in the Warhammer future universe, it was worth the $18 for the campaign so far. I haven't dipped into the Co-op or PvP yet.
I have both of them. I found the original a bit slower, which was fine by me. Prophecy added a new char to play, and if I recall, it made a few minor tweaks.
I really knew nothing about the WH40k lore, but I liked the feel and atmosphere of it. Almost makes me want to take up miniature wargaming again! My wife got a chuckle out of watching me paint a Napoleonic army up way back in the day.