First of all I have no financial interest in the product nor any connection with the seller apart from buying one.

http://www.ebay.co.uk/itm/Nice-Pistol-Grip...3a7276be6e

This is what I have found useful and I will say why. Using a Nikon D800 and a large lens such as the AF-s 80-400mm lens but would also work with Canon or any make using a large lens with a collar.
Convention say when shooting one should have good stance- arms close to the side and support the camera with one hand and the lens with the other hand gripped around the lens bottom to keep a hands free shot steady.

That may well have been true in the past BUT I find that by fixing the pistol grip to the lens collar and revolving the collar to a more comfortable position one can hold the lens far steadier than ever before, almost eliminating the need for vibration reduction to be used. For the cost of the pistol grip anyway surely its worth a punt.
I find that having the grip at about half way between pointing down and at right angles fixed on the collar really does help steady a camera down not only from horizontal but vertical camera shake. Great when using autofocus

Amyway that's my findings for what its worth Ex 2010 Freelander2 GS-e Diesel manual Silver owner

Hi realspeed, I too use a D800 with telephoto lenses. I find it quite unforgiving due to its resolution, coupled with my shakiness! I think this might be worth a try - certainly at that price. I upgraded to D800 from D300 due to layout of controls being very similar (unlike D600). It would be good to leave the tripod in the boot more often and even get the ISO down below 1000 ! Would like a 300 mm f2.8 but Mrs P has a mental block about £4k being good value, even though it would fit on her D3100 ! 2007 SE Manual : Gone and missed but 2013 HSE Lux is better!
Series IIa : Gone but hopefully still alive.

Saved up and sent off for the pistol grip. Ordered at weekend and arrived 1st Class post on the Tuesday - all for £6.99!
I have fairly small hands and wouldn't want the grip to be any smaller but find it light and quite well made and definitely good value for money. I tried it with a 70-200 f2.8 + TC1.7 and it did the job as you say but I would find a slightly fatter grip more comfortable. However, saying that, I can't deny it's good value and can see other uses for it, such as attaching to a flash gun bracket for off-camera use by my photo assistant ( SWMBO or grandson!! ). Anyway, thanks and glad I bought it. 2007 SE Manual : Gone and missed but 2013 HSE Lux is better!
Series IIa : Gone but hopefully still alive.

One other use for it is if you cover the thread underneath you can then use it resting on the ground for those very low shots. glad you think it worth the money, I did as well Ex 2010 Freelander2 GS-e Diesel manual Silver owner