Yes, and it generates a lot of mixed feelings at the gaming table.
On the one hand, the Concentration check for defensive casting was made way too easy for spellcasters, due mainly to every pragmatic mage taking Combat Casting and maxing out their ranks in Concentration. And that's not to mention the fact that defensive casting denies any AoO even if the caster fails the check. This burns a lot of people's biscuits, because they think that makes the whole matter of bothering with AoO's and Concentration checks a moot point. It's just saddles casters with a skill-point-and-feat sink, rather than providing fighters with some reliable way of preventing a harm spell from going off within an arm's reach.
OTOH, the Mage Slayer feat is a pretty ham-handed way to offer characters a means to counter defensive casting, and the +1 to Will saves it grants just seems altogether excessive.
If I'd designed the feat, I'd have it counter the +4 bonus that Combat Casting provides, and have it entitle the attacker to an AoO if the Concentration check fails.
I should add that this debate often amounts to much ado about nothing. Mages can often avoid AoO's with a simple 5-foot step, while conversely any character that really wants to neutralize a caster just drops the sword and grapples.