More like a quick detour to hell with some fast surprises along the way.
Did you like it? Good.
Ready for a trip back? Eh....
The expansion features a rather quick jaunt through the Erebus research complex and eventually back to Hell itself to take out a nasty, nasty demon. Along the way you'll pick up a couple of new goodies: The Grappler which is sort of a tractor beam like gun (nifty, but won't replace the shotgun), the new Double-Barreled Shotgun (awesome, auto-reloads after a shot too), and the new Artifact (sort of a bullet-time effect). The new Artifact is charged by harvesting the souls of dead humans and can be stored for 3 charges. Its useful but I found that I almost always had 3 ready to use at all times because I never wanted to use it (that whole desire to use the shotgun).
The game is short, a true expansion in about 4-5 hours worth of gameply. The disc also comes with Doom II Master Levels pack which makes that cool "Limited Collectors Edition" of Doom 3 that you bought pretty worthless.
If you have any desire to keep playing in this universe I'd say it would be worth it for you to pick up.