Mason has all the markers of aging badly--injuries, reliance on athleticism, refusal/lack of taking jumpers, extremely undersized for the style of game that he prefers to play (PF)--and that's why we were able to get him on the cheap.
I actually have a soft spot for players of his type, and he was actually quite underrated at his peak--he's largely played for mediocre to poor teams throughout his career, but despite his penchance for mostly playing around the basket area, he's actually put up good scoring rates with high free throw rates--so he actually could score, and of course he was a former dunk contest participant and had the ups to yield highlight reels back then as well. He also carries a solid defensive rep, and has guarded SGs and SFs decently in the past.
The Mason we're now getting is one that's really on the downslope though--he's played less than 60 games the past two years, he's lost the body control to finish short jumpers, and he's taking a real backseat in the offense. If he's shooting less than 45% (as he was last year) he's essentially useless offensively because he's a terrible free throw shooter and has absolutely no range (he has a really really funky release that must have scared him off from shooting further). That being said, he's still quasi-useful--he's maintaining his rebound rate, he really spiked up his blocks from the past year, and he still kept up decent defense last year as well, so he still has some glimpses of his athleticism.
Ultimately Mason is really an undersized big man offensively, because while he can score, he has no jumper, few passing ability and few handles. The intrigue is if he runs the floor and procures highlight reels, and his SG/SF defense is also something that could be of interest, although both obviously are not quite what they used to be. He's really a fringe player for us, obviously--he can get the spare minutes from Kevin and Reke at SG, and I'm sure he'll also pick up leftovers from Garcia, Nocioni and Casspi at SF. But I have liked the guy in the past, and I still think he's been a bit underrated in the FA market this year (despite his aging badly), so it's an okay signing.