depends on your list. For a skink cloud, no. ...or..actually maybe.....but no until someone disagrees with me for a combat list with as few drops as you can keep in the 6'' bubble: OH YES. Use the engine to back up your skink priest's lore, and dont use the bound spell except in favorable situations. Ive had good use of it standing between 2 blocks of 8Krox, also covering the skink screen in front. Nitting 1/6 of all wounds in your army is well worth the engine cost. The real question is: are you bringing an ancient anyway? The extra points are well invested if you do, but if you weren't planing to its over 1/10 of your army you are investing just to get the engine, and then you should consider if those points are better put elsewhere...(there might not be, just consider it) See the engine as the cream on top of your already purchased Stegadon cake.
Another +1 to what P-P said. I will add that I believe it is generally a bad idea to field one Ancient Stegadon with the EOTG and one without it. If you have only one Steggy with the Engine that will be the first target for scream attacks, cannons or what have you. If you have two, your opponent may despair of not being able to take out both and may not bother trying to take out either. You probably don't want to spend that many points on multiple Engines if you are packing lots of magic already. I would recommend adding up all the spells your wizards have and adding +1 to the column for every bound spell (such as from an EOTG or Solar Engine) you have. If the number is more than 10 you almost certainly are buying too much magic.
Whoa, 10? I think everything above 6 or maybe 7 is too much Unless 8 of these spells come from are the same frog. The 10 is ok, or maybe 11 (with lvl2 priest-caddy and maybe 1 bound spell)
The thing is, I always ignore Tetto'Eko for these purposes. He can almost be seen as an accessory to your Slann - those reroll 1s are invaluable. He vanguards your Temple Guard. He gives you a spell with no range on turn 1, in a reliable Comet. Beyond that, I don't often care if he doesn't get anything else off! So above 10 is fine in my book if one of your casters if a Lore Master.
I like my EotG Ancient Steg. I use him every game as I have limited units available. Even with a WD Slann and a scroll caddy Priest, I end up with 10 or 11 spells including the EotG. I like my steg because he draws attacks that let my other units get into position better without taking cannon shots. And he almost always takes out some of the opposing army when I use the bound spell.
Oh yeah, the Ward save helps melee. The impact hits help too. So does the bound spell since if you are seeking melee there should be at least one turn when you have lots of enemy units in range to burn.