been using 2 for about 3 or 4 years now. swear by them for sports / walkabout photography as the chances of dropping your camera or having it liberated from you are virtually zero. also reduces cost of replacing lost eyepieces when using neck straps and the camera moves against your chest....done that a few times.
if you need to get out of the way quickly (avoiding cars / rugby players etc) then the camera doesn't get in the way as it essentially becomes part of you. with a neck strap, you'd have the camera bouncing around against your chest, smacking you in the face etc. before buying the hand straps, i'd routinely wrap the neck strap around my wrist when on rallies for this very reason.
should you need to use the camera as a weapon (i go to some 'exotic' places at times), then again, it's in the right place.
only downside with the ones i bought was that the strap could work loose through the adjustment device. this was quickly remedied by using a tie wrap pierced through the strap itself, which then meant that using thick gloves became more of a problem (this is why i have a few of them!) as the device was no longer adjustable.
losing adjustability was preferential to the strap coming loose and the camera accelerating to the deck at 9.81m/s2.
one other thing to bear in mind is that the tripod socket is in a different place on the hand grip compared to where it is on the camera - the grip uses the tripod socket as a mounting point. so, if you're doing tripod mounted panos, then you may want to partially remove the strap so that you are using the aligned camera tripod socket and not the offset socket on the grip strap.
finally, if you're using a heavy lens, just factor in that the camera is in your hand all the time. some people bitch about the weight of the 70-200 2.8L...you soon get used to it
can't remember where i got mine....think they might've been from link delight on ebay.
wouldn't be without them now as for me they're now an essential piece of kit.