Lots of collies here too (not all at once like @PollyRoulson
and only once at the moment)! Also live in the Highlands so they are everywhere and I know collies belonging to friends and family very well too. They are all different characters, but things they have in common:
Herding. Sheep, birds, cats, children, cars. You can (with a massive amount of work) train them to be obedient enough to listen when you command them to not chase, but they still really really want to do it.
Intelligence. Apparently the vocabulary of a 2 year old and the intelligence of a 4 year old. They are cunning and sometimes they manipulate you! This intelligence needs to be exercised, and IMHO this is much more important than physical exercise, though of course a walk will exercise both brain and body. You obviously can't entertain them continually, so you also need to channel their creativity into something like 'creating their own ball games in the garden' rather than 'digging to Australia' or 'shredding the sofa'. Basically they need a job to do. Without sufficient brain use they turn to destruction, fretting, and neurosis...
Relationships with people. They are fiercely pack-orientated. I don't claim to speak for all collies, but I've personally never known one who doesn't know exactly where all the people are on a walk and wants them to stay together. This is good in a way - they are not inclined to running off and disappearing for a day like some breeds. But it may also mean they are determined your children will not be running out in front of you, and will exhibit herding behaviour which your kids may not appreciate!
The other thing is that while some collies turn into family dogs, many are one-man dogs, and may not really be very interested in anyone except their chosen person.
Obedience. Not all it's cracked up to be. They are highly trainable, but no matter how well trained, they still make every decision for themselves; 'do I want to sit and wait on this occasion or is chasing that car worth the telling off I'll get?' They are too clever to be robots, and calculate all choices (you can see it in their eyes)!
I'm not trying to make them out to be bad pets! I love that you can speak in sentences to them. I love that they love to learn new things. I love that they are very affectionate with their chosen humans. I love that they're always on the go and always up for anything.
But I think that you probably shouldn't get one unless you've either grown up with them, or had other trainable dogs before and also have some experience of looking after them/getting to know someone else's. You seem to be thinking along the same lines now yourself, OP, and good on you for asking a question and actually listening to people's answers - you don't see that a lot on MN!! Would you consider getting something else first as practise for a collie next time? And try to get to know someone else's collie meanwhile too? This would be a useful comparison once you've got to know your own dog, assuming you do go for another breed?