I feel strongly that the smooth variety of collie is far less popular than it should be. They are absolutely charming and delightful! My first litter of smooth pups (the "L" litter from Stan and Olinda) was extremely cute and hilarious. They love cuddling and being cozy, even more than the roughs, and are more likely to be found perched on a pillow or burrowed into a blanket that has fallen on the floor.
If you live in a wet climate or a very hot climate, the short coat is infinitely practical. It is also great if you are wanting to try herding with your collie or if you like to go on hikes where your dog might pick up burrs, foxtails, etc. Also, if you are wanting to try showing a collie in conformation, starting with a smooth is the logical choice, as grooming a rough collie for the show ring is quite daunting! Having smoothies in your breeding program is a nice way to see what's under all the fluff...you can't hide anything with a smoothie.
Smooth collies do shed, and unlike the roughs, they don't hold in the loose fur until you get a chance to brush it out. Despite the shedding (which is worse approximately twice a year when they are blowing out undercoat--you will see chunks of loose fur on your dog and can then brush him or her thoroughly), I am totally hooked on smooths, and I predict an increase in their popularity as more people learn that they exist and that they are wonderful!