The time slowing feature is called VATS. When you encounter something you want to kill, you initialize it with a simple button press. The choice to use it is entirely yours.
I was a little iffy about it as well. The first time I blew off a head in slow motion, I was sold. Also, I found VATS much more forgiving than running and gunning. You have accuracy boosting abilities and such for this mode of play.
I think Bethesda did a good job on the game. Character voices were great, the wasteland is very large, and the freedom to do anything is possible. Overall, I was disappointed with the story, particularly the ending. VATS got a little old after a while as well, but I think that was due to me picking up the bloody mess perk. When you blow someone to pieces with a pistol a bazillion times...it kinda breaks the immersion. :D
I don't know you though, so I can only say so much. If you liked Oblivion, you'll probably like Fallout 3.
Person 1: Does Valkyria Chronicles have trophies?
Person 2: No.
Person 1: Forget it. I'm not buying it.
Person 2: Wait! It's amazing! Unique, charming, drop dead gorgeous... Hello?
