I just used the 'official' sony handstrap for the first time and I think it's both good and bad. Good in that it is lovely to carry the camera this way and that if you get it nice and tight, you get a real sense of mass coupling and a strong hold as your hand arches and tenses against the strap.
Bad in that the way it fits is sub-optimal: the bottom plate attachment means that the camera has a higher centre of gravity but a smaller base when set down, and it may not be as good at controlling shake when on a tripod because the contact surface area with the plate is smaller and there are more links in the chain. Also because it is hard to get the strap both tight enough and correctly positioned so as to allow easy use of the various buttons, dials and the shutter. Some tweaking of what i have assigned where might help as will some tweaking.
But overall, I think that when I get used to it, ti will give me better handheld results and in particular I think I'll be able to gain maybe 1/2 stop in handhold speeds...
Anyone else tried one?