He's your average top pair defenseman.
Not good enough to lead a blueline, but would be a luxury as your second best guy.
If the league's pendulum swings back to a slower, cycling game he'll be elite. He struggles in transition and in open, free-flowing games. But he's solid defending in his own end and has a ton of offensive skill--Yandle steals about 20 assists from him a year on PP.
However a true 1D should be adaptable enough to succeed in any situation.
As for his contract, at this point he's worth it. Like many thought, he was overpaid the first year, iffy the second, but from now on it'll look good/like a steal.