However, I find it more balanced than the original Doom 3. Overall, the game is shorter as well as each of its chapters. I started playing a chapter a day, then stopped for several days while I was busy with other stuff and today I played the last four chapters in two runs. This makes it more fun to play, because as you notice you are reaching the end you want to see it as soon as possible (as happened to me).
Then there are some annoying new monsters, but they are not strong ones. There are also some new guns, such as the double shotgun from the original Doom, a Half Life 2's gravity-gun-like thing and The Artifact, which replaces the Soul's Cube; mmm, I think I preferred the latter.
At last, the storyline is so thin that you don't get involved, but it doesn't matter because the game is very linear and thus you can't get stalled nor lost. Just keep going and killing everything in your way.
Summarizing, you'll certainly enjoy it if you liked the original Doom 3. I'm wondering which game I can get now given that Half Life 2: Episode 2 is delayed!