Good Fun
Chances are if you enjoyed plowing through Doom 3, you're going to enjoy the expansion. Overall the gameplay is still just as simple, and just as fun...but some new interesting gameplay sequences have been plopped in to break up the action a bit.
This doesn't exactly fix any of the main problems with Doom 3, so if you were one of the nay sayers of the original then there is really nothing for you here. What the game does well though is gives you some fairly straightforward simple action, there will be very few points where you'll actually get stuck or won't know where to go.
Not one I'll remember for years to come, but a great time killer...be sure to put the effort into playing this game at night, and not during the day to help the atmosphere.