So I bought and mounted my own CGS90 (Custom Grip S90) grip (first blogged here, and found here. More data here).
It's amazing, you would never guess it was not part of the camera from factory. Actually I think it makes the camera more beautiful.
I just mounted it, so I haven't had a chance to use it yet. But I have no doubt at least that the glue will hold: I placed the grip slightly wrong and had to take it off again. And despite the fact that I had not applied any pressure to it yet, and it had had no time to settle, it took all my strength to pull it off...
Update: I've tried it a little now, and it really does make it a lot easier to shot one-handedly.