Based on watching a large number of shooters (new cops), new and current, use handguns under a variety of stressful conditions in a wide variety of weather conditions, I have found that a Spyderco knife is the best tool to work on these slip on grips. Cut the things off and throw them away. The two biggest things these grips do is to make money for the guy that made them and make money for the guy that sold them.
Of course the idea that they may actually cost you in performance and ultimately your life is often lost on officers trying to find gadgets to make up for lack of training or ability. What gets to be most meaningful is the idea that failing to pass our training means a trip home and loss of job. Time after time, shooters facing loss of their job see the light and get rid of those slip on grips.
Many of this genre of grips slide one way or another. Most often they slide up and block the magazine release, preventing full depression of the release and release of the magazine. This is one of the first things we see on the line, followed by looks of chagrin when we suggest that removing the grips might be a good idea.
Shortly there after, the looks of desperation on graded reloading drills when this happens again, seem to be common in every class. On occasion I have seen the grip slide up over the "press" side of the release and now the gun won't retain it's magazine.
By now the affected students are quite distracted over this turn of events. What felt so good in the gun store is now something that the student must worry about. I have even seen students raise the grips to eye level so they can check their grips before shooting. The problem with that is self-explanatory.
Some will now start cutting away at the rubber grip so as to have "clearance" cuts, which start the ripping and self destruction of the rubber. Shortly they have flapping pieces of rubber that cause a pause as they attempt to grip over this flapping pieces.
Then there are the grips that slide down and partially occlude the proper and complete insertion of the magazine into the well. By now many of the students have gotten the idea that the grips are not helping and may actually cost them their job. Most come off then.
Less frequent is rotation of the slip on grip. Shooters index their gun based on how the grip feels and find that gun is pointing elsewhere now.
Finally, depending on the manufacturer of the grips and their condition, they prevent the shooter from making any grip adjustments on the grip, once a hand hold is obtained on the gun.
I see all of these happen on a regular basis. It happens to those looking for some advantage so as to improve their shooting ability. None of my adjunct staff use and all speak against them. The adds another dozen or so agencies.
We don't/can't mandate equipment and how it is set up. Many come to my class with little or no prior knowledge. At least those that are going to rely on their gun to save a life find out about these grips before it is an emergency situation. I can't and won't suggest those grips for any person whose life may depend on their firearm.
Some will say "why don't they check their equipment?" Which is true but not accurate. All of these things take place during dry and live fire training. One would like to expect that they don't have to inspect their handgun for functionality every 30 minutes. The guns are worn under conditions similar to work, both open and concealed carry.
Anecdote follows...
...and this brings to mind the Federal agent detailed to Air Marshall duty, wearing a cotton lined windbreaker with cotton stuck high on the rubber wrapped grips, as the agent walks through a major airport wondering why people are starting to part before him as he trails a couple of National Guardsmen who are starting their stalk on him with M16s pointed.
Avoid them...