I think this is a decent signing. First of all, it looks like the Spurs were only wanting to offer two-year contracts. If that is the case, you can't steal any RFA with a two-year contract. So you have to eliminate all the RFA's from discussion.
Out of the UFA's, Mason was one of the few decent options. The Spurs offered him a three-year contract last summer before they signed Udoka. San Antonio was impressed by his defense and the fact that he can play multiple positions enough to give him some sort of three-year offer, which says a lot to me. Now a year later after Mason added a very capable offensive game ... then it's not too much of a surprise that the Spurs like him even more.
Defensively, I've seen him guard 1's, 2's and 3's. He's best defensively against shooting guards, which is good because that's the position the Spurs will likely want him to defend.