Might go without saying, but keep in mind that the GCD refresh can actually screw you up if you use it with something that has a longer refresh/shorter cast time and the spell isn't actually ready to be cast again by the time it lands.
This is the case with some mezzes, and you'll end up getting the message that the spell isn't ready to be cast, and you'll have to recast. In the end, you'll end up taking more time than if you had never used the clicky in the first place.
tl;dr: GCD refreshes are most successful with short refresh/long cast time spells, and can even cause problems with spells that don't fit this category
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