It's hard to ignore the trend that JA Happ has demonstrated.
In 2014 he posted the best BB rate and velocity of his career.
In 2015 he improved on that BB rate to post another career mark, while adding more K's so that his K-BB% was the best of his career. He also pitched a career high in innings while largely maintaining the velocity gains from 2014, AND he even got better as the season went on. Of course, it all lead to the best overall results of his career.
So I don't really see a signing based on a strong 2015, or a few strong months in Pittsburgh. I see one based on a multi-year transition and demonstrated learning + maturation.
One thing that has to be noted is that, even though he seems durable, he's certainly not an innings eater. This is a 5 or 6 inning guy, which is fine so long as the manager and GM recognize that. Happ doesn't have a lot of tricks in his bag so even common sense tells you that the times through the order penalty should apply to him with full force. By the 3rd time through the opposing team's order, more often than not, they probably have pretty good timing figured out on his fastball.
Aaron Sanchez as a 2 inning power RHP in the bullpen would complement Happ very well. You draw it up as 5 or 6 from Happ, 1 or 2 from Sanchez, then 1 each from Cecil and Osuna.