As a Vampire Counts player that almost always uses two Terrorgheists in my army lists (my regular games are 2000+ points), I can speak from experience when I say that they hit like a truck but aren't that difficult to deal with if you have the right tools. A 6+ Regeneration save as their only defence (behind six wounds at Toughness 6) makes them fodder for Poisoned close combat/shooting attacks, while cannons have a field day against them (not so much bolt throwers and stone throwers though). They are terrible in close combat for a monster and one that should only be sent into a melee against chaff units or massed Archers and the like. Their flying movement is very nice but they need to stay near the General otherwise they will get caught out too easily, while you need to keep them at full wounds for their shooting attack to work. It is poor against Elves and especially units like Warlocks or Sisters of the Thorn, but it is really good against enemy monsters that typically have Leadership 7 or lower. The key is not to be over-aggressive with them as they will fall in an instant against any kind of dedicated shooting/close combat attacks. They are fantastic and easily the best Rare choice but they only really work wonders in the hands of a cautious player, mine die far too frequently because I over-extend with them just to get a shot in!