The grenade launcher can destroy the heavily armored turrets in one shot, even at a low level, but a fully powered-up laser can take it out fairly easily, too. I have used the grenade on some of the bosses 2, 3 and 4 when the opportunity presents itself, and it's pretty effective.
If nothing else, if I'm about to get hit, I sometimes switch to the grenade so that the power of that weapon will be decreased instead of either of my lasers.
Edit: Just saw IGN's score for the game. The game has a few hiccups, but, man, 6.5 seems really low.