i have a pair of chinese 38mm carbon clinchers on my road bike. i love them (although they flex like mad under hard climbing).
in regards to the price difference, this comes down to a number of factors:-
1.) warranty. far sports etc will give a warranty, which they do appear to honour, but ultimately, the enforcement of it is down to their good will. you buy carbon wheels from a uk distributor, and they need to provide an enforceable warranty.
2.) reliability/safety. as above really. if you buy direct from china, you take your own risks. if the wheels fail and your face becomes smeared across the tarmac, good luck with any recouse. if you buy from a uk distributor, you have comeback.
the planet x carbon wheels are really no different to those you can buy direct in china BUT, they have shipped them and got them through customs, they have warrented them, and they have to stand by them in terms of safety and fitness for purpose. if that adds circa £150 to a pair of wheels, that's not bad value in my book.
as for the Zipps, Enves & Lightweights of this world... well that is a different kettle of fish. comparing a pair of chinese carbon wheels with a pair of zipp firecrests is a little like saying that a porsche and a vauxhall are the same as they are both cars.
you make your choices... i chose chinese and am very happy with them. if i had the cash, i would go for Zipps.