I wanted to test with and without minis, but since we have a wet-erase grid permanently on the table, it was natural to put minis on it. It worked like a charm, and it played a lot like I expected a gridless game to play--no one was too concerned about which monster they were attacking, no one ever counted out squares of movement, I never had to stand up and move a monster (I'd just say "he runs away towards the east, and at the end of his turn he's 20' away from you" and the players moved the piece).
I suppose we could try it without the grid, but why bother?