Well you can say that models are rendered faster than blocks or any level geometry, but still you should be aware of the polycount.
A normal glass model or bottle or whatever shouldn't be over 100 polys.
But a normal character (enemie, or any other npc) can easily have around 2000 polys (or even more).
Your main character (the player character) should be most detailed because you see him/her through the whole game.

However, to keep the polycount as low as possible and realize as much details as possible through the skin, is always a good thing.
For additional details you could also use a normal-map, bump-map plus a shader.

regards Thunder