They are very easy to train and thrive on learning but, yeah, you will have major problems if you *don't* train them. They're like borders on crack - ultra high energy so need heaps of exercise, strong herding instinct but with a tendency to nip, dominant personalities with a very strong will (so you need to be very firm and have good training), fiercely loyal and protective and easily bored (so if you don't give them a job to do they'll find one). They need to be with their people - they were bred to work tirelessly with large, semi-wild livestock in company with a stockman all day, every day. When not working they were expected to guard his property and the herd against both intruders and predators.
I just love them to bits and can't imagine having any other dog (Dad gave me my first when I was 6). But that said, I used to help out with ACD rescue and most of the problems came up because people didn't give them enough training, exercise or mental stimulation.