If you have a CX/MTB the south downs way is some of the best cycling I've ever experienced.
The roads round there are also great, if you're riding at a weekend and the sun's out be mindful of the hoardes of angry people in metal boxes heading towards the beach in the morning.
A few years ago a friend tried some of the SDW on a cx as part of a Wriggle ride that was supposed to be suitable for cx bikes. This was near Bignor and he shredded his tyres and had to abandon. It's possible he didn't have tyres that were really suited to it, but in my opinion, large parts of the SDW are just too tough on a cx bike. Others will probably disagree...
The Downslink should be about perfect though, especially the bit near Shoreham. They keep improving it, you could almost get your best road bike over it now!