I loved Doom3, even though it had some problems. I still give it a thumbs up. HL2 had more "stuff" for the player to do and play with, but it didn't have Doom's sense of "being there". Both are great games.
I think the expansion will be even better since the game design team has had time to play the original and address the minor issues found in it. One of the reviews listed said that they saw little of the "take a goodie, bag guy pops out" mechanic being used.