I've usually been stuck in narrative mode, but that's mostly I can't afford any minis whatsoever. However, as of late I did grid off a whiteboard for drawing out maps and tried using bits of Lego for minis.
The minis on the map do help a bit, especially when adjucating spell AOE, and range of attacks, but I can see how you may get a little lazy with descriptions of combat. I found using minis to be rather static, and since I usually describe combat as a whirlwind of activity and movement I find myself resisting the urge to make the pieces on the board move around in the usual dance and movement that would take place in combat. It gets a little wierd describing the effects of damage, the lunging, the dodging, the feinting when two pieces sit on the board side by side for the duration of the entire combat. I mean really when the Hill giant hits the dwarf for 14 points of damage, don't tell me he's still standing in the same 5' square.