Duncan

Theorycraft says: Remove "or UnitMana('player')>XXX" and it should still do the GCD check and the check if the spell is usable or not. I think the last part checks if you are stunned or generally unable to do something, before it cancels the form?!?

So, without testing, it would look something like that:

/run local f="Cat Form";f=GetSpellCooldown(f)>0 or not IsUsableSpell(f) or CancelPlayerBuff(f)
/stopmacro [form]
/cast [stance:1] !Dire Bear Form; [stance:2] !Aquatic Form; [stance:3] !Cat Form; [stance:5] <Tree of Life Form / Moonkin Form>; [nostance, flyable, nocombat][stance:5?6] !Flight Form; !Travel Form

E: Might only work with cat form, though, due to the fact that the first line only checks for Cat Form. The other forms should be added there as well.