It was implemented reasonably well, there's no full team behind the screens like at the World Cup and I don't think linesmen were asked to keep the flag down when they're not certain either so this tournament is actually a good step forward from the way it's used in European football.
In most cases players' reactions were very subdued like after GLT came in which eliminated the grounds on which to discuss things with the ref.
However as always technology is rarely enough to completely erase the chance of human ineptitude fucking things up and so even looking at a replay which clearly shows what occurred refs often refused to correct their previous calls out of pride or other dumb reasons.
Even worse and in my opinion the biggest room for improvement is the footage that's presented to the ref. Most of the time for some reason it's not the same as the footage on TV which usually shows an incident from different angles - in the domestic cup final for example when there was a big call for a penalty in injury time the screen kept showing the same 1 sec scene where contact is made over and over instead of showing the full scene. Looking just at the short clip you'd say it's a stonewall pen, the bigger picture however showed after the foul a different player got the ball and got his shot off at goal which was saved ie not a penalty. This type of bias from the bloke in charge of the video is obviously going to lead refs to make certain decisions but it looks like they're not making the same mistake in Russia.
