I played against a Mastodon in a 3k game a couple of weeks ago. Whilst it was a fantastic model and looked awesome on the table, it didn't really make its points back. It killed a couple of attack bikes and a few tacticals. It also managed to immobilise my charging Leviathan with overwatch from one of its lascannons - very lucky rolling and shouldn't have happened anyway because after the game I read the rules and it turns out defensive fire only works when there's a unit still occupying it. But I guess this error was made up by my opponent only rolling (and failing) to restore his void shields once.
First turn I did ~4 hull points because it was pretty much the only thing on the table. Turn two everything disembarked, so for a while I was more preoccupied with it's contents, plus a load of other deep-striking and outflanking units that had also just arrived. I think if I'd continued trying to kill the Mastodon instead of the rest of his army, it would have done really well as a massive distraction Carnifex - not doing much damage to my army, but soaking up a load of damage and saving his more squishy units. But I didn't fall for that trap. Turn 4-5, I took off its remaining hull points by charging it with a load of Gal Vorbak. Turns out a rending S10 powerfist, rending S7 power mauls and a ton of rending S5 standard attacks can do a lot of damage.
Fun game, but with the Mastodon, it felt like my opponent was down 300-400 points.