Grip increases centerspeed by 2%. This has been acknowledged by even MLG level players as "barely noticeable".
The ACOG, in addition to providing zoom also increases centerspeed by 2% (gives a free bonus grip effect).
Centerspeed, is literally, the speed at which the gun will return to center. A gun may fire its next round prior to being completely centered, thus leading to a trail away from the original gunshot. Therefore, higher rpm weapons benefit much less from the effect as less time is given to return to center.
As JSlayer says, an Evo with Rapid Fire will 'kick like a mule' even with the attachment, though it is an unfair and extreme scenario. Regardless, to get any measurable benefit it has to be on an automatic weapon with a lowered RPM and over a 'long distance' will obviously become more noticeable as any increase/decrease of accuracy is visually obvious over longer distances (farther/short distance between bullet contacts).
This is why many will report the Peacekeeper showing an advantage to the attachment, as it's a low rpm, high range SMG.
That being said, the answer is it is completely useless on most guns and fairly useless on a small number. If you 'feel' it is having an effect, then who cares, use it. However, based on testing and variables involved it is, without any real argument, a low value attachment (based on the pick 10 systerm).
Can we stop making these threads now?