I use one on track only.
I'm not gonna claim that it makes a huge difference to the overall driving experience but it does keep you located in your seat quite well and not shifting about so much during cornering and hooning!
I suppose the acid test is that whilst whizzing around a track, you wouldn't even know that it was fitted as it is not intrusive in any way. I think it is benificial and relatively cheap, you just have to make sure you fit it carefully as to minimise wear/fraying to the seatbelt.
Oh, and when removing your seatbelt after use be
very anal in assisting the reel retraction back in as you will scuff and damage the driver door CF panel each time otherwise!
Hope this helps?
