Can you advise me on the nature of this breed please

I love them!  However I will not have them while I have other dogs of both sexes.  They are VERY dominant dogs and WILL be the top dog over any other.  They are VERY intelligent, NEED mental and physical stimulation, the type of dog the will get into trouble if let become bored.  Puppies MUST be tested for deafness too.
Dawn.

Ditto what Dawn says, they do vary a lot in temperament, so do spend some time with both sire and dam.  My boy has a good temperament for an ACD, but if he was a Lab you would shoot him!  He will not back down from confrontation and doesn't like being hassled.  He loves the people he has known for ages but will generally ignore strangers.  He is good with my kids but I won't trust him with visiting children.
Saying that, I have an older male dog, a XLab, who will sort him out if he gets too lairy so they can live with dogs of the same sex, but you have to really know your dogs, I took a bit of a chance I suppose but it has worked out, I couldn't have had a bitch as my current XB bitch can be very feisty with other girls

If you need more info, look at the www.acdsocietyofgb.com website