Martin has good size at 6'2'', and was ranked the #26 North American skater by NHL Central Scouting. He brings a left-handed shot to the Sabres system, and is more of a reliable, smart defenseman than a flashy player, but he's a good skater and puck mover, two things becoming more and more important to players on the back end.
Martin brings good value to the Sabres, as many had him projected to go in the first round earlier this year. He had a rough end to the season, but has all the skills - both offensively and defensively - an NHL defender needs to be successful. Whether he can live up to his earlier projections (or why so many other teams passed on him) remains in question, but if he can put his poor finish behind him, he could turn into a steal for the Sabres.
Here's a scouting report from In Lou We Trust:
Martin uses his size to his advantage and is most always in the right position to make a smart play. He makes good first passes and is working to become a stronger two-way defenseman. He is projected to become a top four defender with all of the strong tools he possesses. He is also a strong asset on the power play.
On the negative side of things, Martin didn't play particularly well at the NHL Top Prospects game. His most notable contribution was causing a penalty shot that nearly turned into a goal when he Nikolay Goldobin down from behind. He also slipped six spots in the Draft Prospect Rankings (from 20 to 26). That is mostly due to his play in his own end. While he has been trying to improve it, he finished the season a -16 though he was the second highest scoring defenseman with 37 points in 2013-2014.