Martinez has really bad press but he is a great goalkeeper.
The ammount of really weird takes on him here are quite strange
1. He ain't an off the charts "cat" under the posts like De Gea, Casillas or Fillol...yet he is very good and compensates his lesser agility with better positioning in order to not suffer as a shot stopper, but he is not the one for the impossible jump in the very top corner, he would try to read the situation to avoid the need of such actions.
2. His best asset is that he commands the little area, he is agressive on the ball, like an old guard keeper grabbing the ball most of times, cutting crosses all the time with lots of presence and continuosly keeping his defenders on alert. He is way better in this aspect than the mentioned before and perhaps the best in the world at it, which for me is the best asset with any keeper to keep things calm. He is a very strong fella BTW, you can notice thsi in these actions adn even in penalties how he can improvise with his other hand without loosing stopping power.
3. He is great on one of one situations, he has great positioning and anticipation and even if he is constantly trained in Villa because of how easily teams can have chances against them, in Argentina he tends to keeps thing tidy with the few ocassions the rival attacks and when he fails, he doesn't forget himself and works harder.
4. He is good on the ball, not Ederson level, Neuer level, but every day getting better and more confident, in fact already knowing Licha it's an advantage if he had come to United. Recently he has add some very long passes with great precision out of nowwhere, in this recent Copa he already made at least 3 or 4 assists that Argentina blew putting all of a sudden a player against the rival keeper, it looked like he has been practicing that stuff.
5. He is a looney on the best sense, the best keepers have some sort of lunacy and big presence that intimidates rivals and gives confidence to his mates.
6. And of course a great penalty saver...
All the antics are in the pitch, outside of it or when the match ends, he is the ultimate pro with a big winner mentality.