From a technical standpoint, VAR works extremely well.
It's let down by ridiculous laws and poor judgement. The offiside rule is a joke as it stands. I agree that you have to draw the line somewhere but, for me, it has to be from the last part of the attacker's back foot. Armpits and the very back of the heel are pathetic reasons for disallowing a goal and totally against the spirit of the law in the first place.
Nor are the officials helped by wanker players throwing themselves down at the slightest touch and commentators/pundits claiming 'well, there was contact so he was within his rights to go down'.
As for its use, I think, if they're serious about not re-refereeing decisions, then both the VAR official and the ref via his monitor need to see the incident at full speed only and if they can't make their mind up based on that then the original decision should stick.