I've been using the Garmin mount (in prototype form) since July of last year. I rode about 13,000 km's with it, right across North America, and had no problems.
The locking mechanism seems pretty secure to me. I'm not sure if your question addresses security in the sense of "holding the thing in place" or security in the sense of "anti-theft", so I'll try to answer both questions.
Regarding "holding the thing in place", I feel very good about this aspect of it. The sliding mechanism that goes up and down at the top of it drives a flat pin (sort of like the end of a slotted screwdriver) into a receptacle on the back of the GPSR. You can see this receptacle if you look just below the Philips screw that is at the top, middle area on the back of the GPSR.
Any upward, downward or sideways G forces that may be encountered are absorbed by two pretty solid round lugs on either side of the mounting bracket, at the same level as the flat pin. These round lugs fit into the two round holes at the very top back of the GPSR itself, so neither the locking pin or the big flat bar across the top that actuates the locking pin are subject to any forces arising from inertia of the mass of the GPSR. In addition, there is a small security screw that holds the locking pin in the down (locked) position, if you were planning to do any serious off-road riding, and you lowered the bar and then secured the security screw, I think you would lose the fillings from your teeth before you lost the GPSR from the mount. But, having said that, the weak point in the case of serious stress (e.g. dumping the bike) would be the 4 bolts that attach the GPSR bracket to whatever you have mounted it to. For what it's worth, I flipped my ST1100 upside down in a ditch two years ago - front portion of the bike came to rest on the SP III GPSR, in the Garmin bracket, because the windshield busted off - no harm done to the GPSR or the mounting bracket or 4 bolts, which were the same specification as the SP 26xx bolts, just a bit shorter. Deceleration G forces will not be a problem, these will press the GPSR more firmly into the mount. A really over-enthusiastic clutch drop on a Hayabusa would put stress on the flat pin, which is the only thing that resists acceleration forces, but I think you would probably lose anything strapped on your rack, pillion or panniers before you lost the GPSR.
Concerning security from an anti-theft point of view, it depends a lot on what kind of society you live in. I never removed the SP III from my ST1100 whenever I parked it in Switzerland over the past 2 years, the only exception being if I was parking it on the street overnight. But in public lots, restaurant lots, etc. I would just leave it in place, and the SP III bracket just had a thumb-screw to hold the GPSR in place. The SP 26xx bracket requires a special tool to remove the security screw, it does not have a thumb-turn on it. So, this is more than enough security for me if I park anywhere in Switzerland, in rural Canada, or in an indoor or guarded lot anywhere else. If I was leaving the bike on the street overnight in Poland or Bosnia or Merseyside, I would probably remove the GPSR from the bracket - why tempt fate?
Hope this helps you decide.
PanEuropean
