Well, 4th edition assault rules say that if you shoot before you charge, then you lose your +1 attack bonus.
Photon grenades say any unit assault the unit with photons loses their +1 charge bonus.
Well, then, say a squad of 10 will usually get about 20 attacks in a charge. Since the photon grenades would cancel 10 of those attacks, i think now in 4th edition, that ten man squad will elect to shoot before assaulting, which will still give them 20 attacks, since they know they'll be losing 10 from charging either way.
This then cancels out photon grenades and it's usefulness. Kinda sad too, because they were really useful against me, as a DE player.
Of course, photon grenades are still useful against troops that don't have a shooting weapons, like warp beasts, some tyranids (i think), and perhaps orcs who suck at shooting anyway.
But, we can all see photon grenades are not going to be as useful as they used to be. Hopefully GW will do a chapter approved to modify them a little. Maybe changing it to those assaulting lose their two weapon bonus.. .or even, that if a unit shot at them, they only get half their attacks.
I'd hate to see a useful upgrade get shot down with the new rules. Any opinions?