personally Im a fan of igs with great weapons. I think the strength is worth it. people usually dont like to give away the extra armour save but really its only one point of less armor save.
I’ve found that with M3, no one engages them until they are whittled down enough to be manageable. If your opponent has any redirectors, this unit will never see combat.
if you’re gonna play them in one unit then there may be problems with re directors and stuff like that, mind you there is still an army around the unit to deal with redirectors. but in 2 units I think it will be a good investment and even though they have Movement 3 they still have an average charge range of 10". its not incredibly easier to avoid compared to a M4 model. your average M4 guy has only 1" further charge on average.
Blunderbusses, so worth it. Specially if its a multi wound unit coming at you, hit them with ash storm then shoot the hell out of them with flaming banner blunderbusses. Even if they are single wound the sheer amount of fire you can put into some one, gods help anyone who tries to charge.