I've had some experience of Nexens. When I bought my Astra it had some very old Michelen Primacy HPs on the front and Nexen N2000s on the rear. The michelens needed changing almost immediately after buying the car, so I had the nexens moved to the front and put new toyo t1-rs on the back. Despite the Nexens still having around 4mm of tread they were nowhere near as good as the 2mm Michelens they replaced. The traction control was kicking in noticeably sooner after making the switch and they caused more understeer in the wet.
I changed them when they got down to 2mm for new Toyos to match the rear and again they are much better. Traction control kicks in much less frequently and the wet grip is much better (which is saying something given the toyos arent paticuarely good in the wet).
I know the N2000s arent the same as the CP641s but from looking at these reviews the ones I had are actually better than the ones you are after. Based on that I would say avoid.
http://www.tyrereviews.co.uk/Tyre/Nexen/CP641.htm
http://www.tyrereviews.co.uk/Tyre/Nexen/N2000.htm