[Kalin]

Not only do you get the gem cost back (isn't it 55 gems?), which is easily done in a large game of back and forth battles, but you also get a ton of regenerating meat shields for your armies... and they can go into water.

If you're already an aquatic nation, then sea trolls might not be as great, but if you're not, they are a great deal.

You tell me, what other easy summon actually gives you back it's gem cost? Even if it doesn't fully return it's initial cost, everyturn that goes by with the sea king alive, the gem cost of the summon is effectively reduced by one.

If you're playing a quick rush style game, you might never even get the spell researched, so there's no point talking about it. If you're playing a medium game, then sea trolls can be a very cheap summon if you split the kings out (to refund yourself and cast more later). In a long game, the kings work like clam shells for pearls. Sure, they aren't as effective, but with the meat shield involved, I don't see how you can say that it's not a good investment.