I have a Jack Russell x Maltese Terrier, although I would say he is 99% JRT. When he was a puppy he was EXTREMELY hyper, and during the day he would just zoom around the house and wouldn't stop!
He has a fantastic personality, and is also very sensitive (caring), but can be quite stubborn. JRTs are a very smart breed though.
I've also owned a Kelpie x Rottweiler. Kelpies are great dogs but they need lots of space and need to be kept busy as they are working dogs. I don't really think full Kelpies should be kept in the city as they need huge backyards. They are a really smart breed and have a really funny character too.