Depends where you are, like most troops...
healers are also useless at higher levels, but at lower levels they are crazy! Dragons are OP down at 7-8 for war.
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Mine are only lvl3, but on the right base, I've had them go through the core, and out the other side - like all troops, they work better on some bases than others..... (excellent at taking down multi mode IT's, as well as a good cc troop for defense)
Plenty of youtube videos featuring them, if you bother to look........
they should reduce the training time that will make them more popular again
Don't rage on my post, but valks are probably the best troop in the game. Using 1 golem, 5 valks (Level 1), 20 wizards (Level 5), and a few wallbreakers, I got 82% on a maxed TH8 for 350k gold, 400k elixir, and 1.5k DE. Every other GoVaWi that I have done after that (6 more in total) has gotten me 3 stars. Learn how to use valks. It pays off.
Lured by CC
http://i61.tinypic.com/r2uy4y.jpg
4 seconds later, lured by heroes
http://i61.tinypic.com/6z93kp.jpg
5 seconds later, breaks through walls, buildings destoryed as collateral damage, heroes going down like nothing.
http://i62.tinypic.com/2n9grjl.jpg
13 seconds later, core is done with, troops are cleaning up
http://i61.tinypic.com/10oma78.jpg
22 seconds from moment valks meet CC troops till when core goes down. Finished that up wih a total of 74% 2 star
http://i58.tinypic.com/2z3p2f9.jpg