With the Siphon skill line being back on the GCD, it's actually pretty awful. Aside from the fact that players have effectively been taught to cast it off the GCD for a bit now, it also causes problems when you effectively are losing a GCD trying to trigger the siphon. That's a massive DPS loss through no real fault of the player, and that's an issue. I know Zos has worked hard to improve the functionality of the skill, but honestly, it still isn't there. It's still a clunky, wonky skill that works about 40-50% of the time on average. I think we need to find a reasonable compromise that meets the improved functionality (because it has improved) and limits how much free damage it's giving without making it a "slot it and forget it" passive skill because the functionality is still not good enough.
Give it a cost when it connects. If it doesn't, you don't lose resources. If/when it connects, it has a Stamina or Magicka cost but remains off the GCD. I think this is a much more effective solution to the problem until a better targeting system is found for the skill in general.