While initially a bit skeptical, I have added Grip Force Adaptors (http://gripforceproducts.com) to my Glocks and am truly impressed by the benefits they offer, as noted by Todd and others.
As a long time 1911 shooter, when I press out a stock Glock, particularly the G17/G22, I typically find the front sight sitting too high and have to consciously cock my wrist down. The GFA slightly changes the Glock grip angle, resulting in a perfect natural sight alignment when I press the pistol out. The GFA beavertail helps to ensure a more positive grip when aggressively drawing the pistol. It also provides more leverage which seems to help control the pistol during recoil on fast shot strings. The timer indicates I am having both a faster draw and quicker splits when I use a GFA equipped Glock. The web of my shooting hand occasionally gets bitten by a stock Glock slide--especially during high round count training sessions; the GFA beavertail seems to eliminate this possibility.
Installation is quite simple and takes about 60 seconds. I have modified all my GFA's. On the G17, I just reduce the ridges on the underside of the beavertail using a Dremel tool. With the G19, I do the same ridge reduction, but also cut the grip portion in half. All the 3rd gen Glocks I use have been stippled, so the GFA's on those pistols also got stippled.
On RTF2 pistols, the GFA’s built in texture works fine, so I modify them as described above, but without sippling.
All in all I am quite pleased with the GFA and now consider it a desirable addition to my Glocks.