I use them pretty much all the time ( a rare one on one fight, I might skip).
Here's what I find valuable about using the grid:
players pay attention more because they have something to see and think about
players who aren't paying attention get back in game faster on their turn, because they can look at the board and see the situation, saving the DM time from re-explaining who's where and where the bad guys are.
it keeps players from being everywhere at once. In games without grids, I've got one player who would be simultaneously retreating from an area effect spell, fighting a bad guy, and trying to scoop up the treasure dropped across the room. The grid puts them in a specific location relative to everyone else.
I can show the 3d models and mini scenes I've put together for the session. We've played on a 28" Ship (The Maiden of the High Seas from WorldWorks Games, very cool). 3d dungeons, etc. I like making stuff, this is an opportunity to put it all together and use it.
The grid makes it easy to adjudicate area effects and attacks of opportunity and flanking (no arguments, just look at the board).
Janx