I like the Decimator and I use it very often with Zerkova and EIrusk. Any caster that can affect your enemy's movement will like the decimator. I don't get why people don't like the gun, it's very powerful for a ranged weapon, with a little utility mixed in. The decimator is made for targeting high value target, the ripsaw is not exactly designed to deal with infantry and neither is the gun. It's a jack made for killing jack and beast, and the gun is doing a very good job at weakening them before you get to melee.
I don't think it can be compared to the gun carriage. Yes they are both 9 points but the comparison end there. The gun carriage is utility/anti infantry, while the decimator is a heavy killer, yes the gun carriage is faster and can create terrain, but when was the last time you downed a heavy beast in one turn with your gun carriage? They don't fight for the same spot in a list and I think they work very well together, rough terrain to slow the enemy can give you one more turn of shooting or actually allow you to charge with a SPD 4 model without reach.