Originally Posted by Andy Taylor

Magnaporting reduces the value of the firearm. As does any custom modification. It might increase the value to a few select purchasers who plan to have it done to the gun, however even with these buyers, the increase at most is the same as the cost to have the porting done.
It does reduce felt recoil
It does redirect muzzle flash, into the field of vision of the shooter. Some do not mind this, some detest it.