I keep reading about the magic round improvement of the 9MM +P+.
In what ways? None that I'm aware of.
Does the recoil go up with +P+?
Of course it does. More muzzle velocity and likely greater powder charge mean more free recoil energy. It's basic physics. Whether the difference is enough for you to perceive is another issue only you can answer.
Do the barrels suffer from +P+?
Of course, but the difference is likely de minimus for a typical user who carries +P+ and only fires enough such rounds to prove them functional in gis gun.
What do the gun manufactures think of the use of +P+
I'm unaware of any gun manufacturer that sanctions the use of +P+ ammo. When you think about it, to do so would be foolish, because there are no standards (upoer pressure limits) for such ammo. What does your owner's manual say?
In choosing defensive handgun ammo you want a round that feeds reliably, and penetrates adequately while, ideally, expanding reliably. Most premium JHPs (eg, Federal HST, Speer Gold Dot, Hornady Critical Defense) will do these things in service calibers. Check terminal ballistics performance -- published by the manufacturer or by independent testers on the internet -- to make sure you the round you are considering carrying penetrates adequately (eg, at least 12.0 inches in calibrated bare, 10% ballistic gel per the FBI, or 12.5 inches per the IWBA), and expands well. For 9 Luger, the heavier the bullet the better the performance in general. It is not uncommon to see light-bullet +P and +P+ rounds that fall short in penetration, because the faster velocity increases expansion, and expansion retards the most important aspect of terminal ballistics, penetration.
I think +P+ is a marketing gimmick meant to seduce gun owners than to stop bad guys.