I have a border collie mix (mixed with black lab) and up until her senior years when she slowed down some, I'd say she was high maintenance. I'd only recommend them to experianced dog owners who have the time and energy to devote to their training, exercise, and care. If you have that time and energy though, they are well worth it.
Keep in mind that they do "nip" when herding. It doesn't usually hurt much but it can startle some people, especially children.
Also they are SMART. They can and will get into anything and everything (I went through several trash cans trying to find one she couldn't get into, eventually had to give up and DB built me cabinet to store my trash can in), and are pretty good escape artists too (mine could open a chain link fence gate unless we had it either tied off with rope or padlocked). This is one reason why they need so much training, if you don't give them something to do, they'll find something to do on their own and odds are, you won't like it!
Overall they have an exellent temperment though. They LOVE their owners. I won't count on them for protection though. While they will bark to alert you, they aren't always very brave. Mine hides behind me when she is scared, hoping that
I will protect
her!