I will mirror what Mark has said. The beauty of KW, that as far as I can tell no other passive suspension kit can offer, period, is that on the road it provides a compliant ride, far more compliant that the spring rates should provide, but is stiff when it needs to be - you can really really push into a bend and the suspension just keeps on working. I have stock ARB's and the body roll is kept under control so well that I have now scrubbed plans to upgrade the ARB's. You cannot tell the transition between the different damping rates it just happens seamlessly. If you are looking for a more track dedicated car then i'm sure some of the other aftermarket options are as effective - however probably require a tad more effort and time to tune because they don't have that flexibility the KW's have. I'd say that KWv3's greatest strength is their all round flexibility. If you want something more focussed and specific, even though KW will run with the best of them the gap is significantly narrowed.
They are expensive relative to other kits, but the quality is far better than anything i've seen (stainless steel construction). And relatively speaking I don't think they are that expensive. You can spend alot more on a mountain bike which would only really last a couple of years.