Doesn't Halo sort of have classes in Reach? Not customizable ones, mind you, but they still have different starting weapons and abilities. I like those sort of classes because you can start with whatever weapon you like best, but all the really good weapons need to be picked up, so it still has balanced gameplay.
If you are talking about customizable classes, then those are bad. They prevent new players from being able to compete with veterans. And I know, veterans will know where all the best weapons are to pick up, but it is much easier to learn where weapons are than it is to build up a character to have the best weapons and equipment.