Are they just to be used in certain circumstances i.e the person has been spotting at them? In that case I support it and they shouldnt spit. Serves them right.
If every arrested person gets one put over their head , then no.
I think it is a good idea in principle - as long as isn't taken out of hand in practice and used to subdue anyone who could spit as opposed to those who have already spat.
I don't give a damn about someone that a police officer is seriously worried will spit or bite them. Stick them in a bloody unicorn hood for all I care.
My worry is a bit like tasers. When they were first introduced, they were presented as an alternative to firearms that would be used in similar circumstances - not to solve more situations, but to solve existing situations in a less-lethal way. Move on more than a decade and you regularly see them being deployed for things the police would never think of using a gun for. They've become a routine alternative to restraint and are now being fired at around 2,000 people a year (and pulled on about 8,000 more).
I don't want to live in a society where arrest almost automatically means having a bloody bag placed over your head.