Yep they're probably my favourite breed. If I ever get another dog in the future it will no doubt be a Border Collie. Our dog is a mongrel - hard to say what exactly but he definitely has some border collie in him and maybe some sort of terrier. It's a case of can't teach an old dog though - although he looks older than he is :P
Edit: I watched a documentary a while ago which included a part about this Border Collie (in Germany I think) which is supposed to be the most intelligent dog in the world. It has been taught to retrieve any toy the owner names from a whole pile of toys (like 100 toys or something) and it can recognise the toy if you show it a miniature version of it or even just an image of it and it goes into the next room of the house and fetches the correct toy! I was looking for any YouTube clips and couldn't find him but I found a video of another one which has been taught similarly (apparently just on a smaller scale);
http://www.youtube.com/watch?v=pf0Vr0MSdHg :)