I had, briefly, an ACD as a kid before it went back to the farm from whence it came after it quickly became apparent that it needed more exercise and room to run than a two working parent subruban family with young children could afford. He was a really great dog personality-wise -- just got destructive when bored and under exercised.
They have a lot of energy, not necessarily a problem if you've got the time and the space for them. The only thing I know that I would caution you about is that, as is suggested by their name, they were bred to herd cows and do so by nipping at their heels. They do this by nature, not training...the one we had would herd us children around using this method. I guess cows don't mind this(?), but many horses would, so you'd need to train the dog not to do this with your horses. But otherwise, they're probably a great farm dog...that's what they were bred for.
I think there are several ACD owners on this forum... Puppy & Dog Forums
. They could probably give you more detailed answers than my general knowledge/limited experiences.