I have been looking at them too, but I think the price will prevent me from getting one. I get some head shake under hard acceleration, but no tank slappers. If it ever got that far, it would push me to buy one.
I have heard that they greatly reduce fatigue off road, by doing a lot of the work for you. For me, that isn't enough of a selling point, since I don't get that worn out riding. For someone who rides really hard, it might be worth the high price.
The good thing is that the main dampening unit can be used on future bikes, and all you would need would be the bike-specific mounting kit.