I don't have time to read all of the replies, but I'll give my experience. My parents have a red heeler. They got her from the humane society, she had lived on a farm previously and was about 5 years old. Sidney is a great dog, but she's quirky. ACD's are really, really smart, but not in a Border Collie, "what can I do to please you?" way. They were bred to work independently, so they tend to question you a lot. We like to joke that when you ask Sidney to do something, she first takes it to committee.

They can also be hardheaded, as they are bred to work cattle, which are more stubborn than sheep. (Not to mention bigger.)
I love them, but I wouldn't call them easy dogs. Sid is also very prey-driven, and chases cats, squirrels, anything small and furry that runs. Skunks...twice, actually.

Did I mention they're hardheaded? LOL.
She's a very sweet dog, though, loves my parents to death is great with kids. She's also very territorial about the house and the property and prone to alert/guarding type barking. She tolerates Sienna (my BC-type mix) but snaps to correct her if she gets too hyper, or otherwise offends Sidney. ACD's can be mouthy, they herd cattle by nipping at their heels (the 'heeler' part of their name), so it's an instinctive reaction to nip at things. Great bite inhibition, though.
They get very attached to their people, but don't necessarily like a lot of physical affection. Sometimes called "shadow dogs" because they like to be near you, but not on top of you all of the time.
My aunt has a blue heeler / border collie mix as well. Her dog is also really smart, independent, stubborn, territorial, and has personality plus but the BC part softens her up a bit. She gets along with cats and is smaller and has a 'softer' personality than Sidney.
Anyway, that's my experience. They are neat dogs, but not for everyone. Good luck with her, she's adorable!