Lately I've noticed that a lot of people are going from large (BCs, belgian shepherds, etc) to medium or small category. I think that especially medium category is getting popular over here. There's not that many handlers, but there are some of the best handlers in the country running medium dogs. It's interesting that Linda has got a sheltie. I'm curious how they're gonna be.
As for purchasing new puppy, I'd also rather choose dogs that are "proven". I don't know about US&Canada, but it's kind of tough to get your hands on sheltie w/ good personality and temperament. I've met too many shelties that were fearful and/or lacked drive. I wouln't leave out show dogs, but I'd watch out for dogs that are succesful and good at the sport. We have one dog over here who has never run agility, but he's father of wonderful group of shelties who have icredible speed and great drive.
Mine dog's father also never stepped into the ring (he's actually grand champion at dog shows) and even when his mom competed few times, I was told she's nowhere this lively, fast and wild. A lot of people told me that my dog is a one million dog and I should cherish him, because there are not many shelties with such a great friendly attitude and happy open temperament. Though, it was really just luck on my part. I knew I wanted a sheltie back then, but I didn't know what to look at and I had no idea that some shelties are so timid or w/o drive. I didn't even know for sure if I would do agility, but when we arrived to breeder to pick him up, she handed him over and told me that I need to give him a chance at agility that he's going to love it and be great at it. Later on, I learnt that not all shelties are like my boy and since then I've been watching all shelties around, learning about different litters etc.
The point is even when I was lucky for the first time, I wouldn't do the same thing again. If I decide to get another sheltie, I know I'll look for a puppy w/ certain traits, so the chance of getting another happy agility dog is higher.
EDIT: I should also mention that I believe that owner plays the role, too. I know some people who have dogs from different breeders, totally different parents and background and they still project similar behavior (usually unwanted).