as mentioned, genuine is the only piece of mind guarantee. especially if you are hard on them. even at the prices I sell these at they are still relatively speaking, expensive.
if the vast majority of your driving is on road you will be fine with a reasonable after market CV joint.
ive had a blue print number from the local motor factor on the nearside for over 100000 miles, and, dare I say it, a HDK on the off side for about 40000 miles.
I spent months trying to source direct from GKN as an alternative option, in the end I gave up..
I am working on an other option at the moment though..