First, thanks to DYH for kicking off this discussion. I'm David Block with HandAble. It's completely understandable for folks to have apprehension about using some of these devices on there expensive mobile devices. Products like Flygrip, Thing Sling and HandAble are fairly new to the market, and there is (in our experience) a bit of a learning curve to understand how they work, and how secure they will be on your devices.
Speaking for HandAble, we have spent an enormous amount of time (nearly 2 years in our case) making HandAble as secure as can be, while making the removal of our product fairly effortless, leaving NO residual adhesive on your phone. We use a 3M product that has a Very High Bond, but comes off with no mess.
Our adhesive will work on your phones back, and most cases. There are some materials, though, that pose a problem, such as Gel Cases which tend to have a 'silky' finish. But, if your case has a smooth hard shell back, your good to go.
I'm sure the good folks at Flygrip and Thing Sling have put their products through the same rigorous testing that we have. And, after almost 2 years of development we're confident that HandAble is as comfortable a feel, and secure a hold, as you'll get in a mobile device holder.
If you're in Southern California, you can find us Sundays through the holidays at the Orange County Marketplace in Costa Mesa.