I think for me it didn't feel overly effective. Perhaps part of it was poor luck - I think I only ever made two or three creatures fall unconscious. I played a wand wizard, so that made it less useful as well.
Additionally, I'm not a huge fan of the "slowed" condition. Ranged attackers aren't particularly bothered by it, since mobility isn't as much an issue. It's great for keeping enemies at bay... but then, your defenders are going to run right up to them anyway. It's good against highly mobile enemies, but that's highly situational. Personally, I'd rather have something that will be useful independent of the enemies I'm fighting, especially in the early levels when resources are limited.