The Shetland Sheepdog has been described as a miniature Collie for its entire history as a breed. There may be some similarities, but the Collie and the Sheltie are completely separate breeds of dog.
The temperment and health of the Sheltie is dependent not only on the owner taking responsibility for his/her dog, but primarily on the breeder. Well bred Shelties are bright, intelligent and devoted to the family. They are naturally suspicious of strangers and make excellent watchdogs.  The Sheltie loves to play and is great with children, the breed having a level of stamina that will make mom and dad take bets as to who is going to get tired first. The desire to please is very high in the Sheltie, and therefore makes this breed excellent in obedience, as well as herding competition and agility work.

BUT when a Sheltie is poorly bred, be it by a puppy mill or a backyard breeder, who either don't have the knowledge needed, or they simply don't care about the breed, their only concern being making money, a multitude of problems can occur.
And it is usually the family that has adopted the dog, and fallen in love with that puppy, that has to deal with the problems that develop.
The poorly bred Sheltie can be timid to the point of fearfulness. The danger of fear biting is great, and can make the dog incompatible with children.  Some will become yappy, high-strung and nippy.

When purchasing a Sheltie puppy, be sure to talk to the breeder about medical problems within the kennel's bloodline.  Should the breeder say that there are no medical problems in his/her kennel's bloodline, be very skeptical and find another breeder who may be more honest, and not so interested in a sale.  A reputable breeder is aware of the medical history of the dogs they are breeding, and the genetics involved in trying to improve the dog.  They will be honest and up front about the weaknesses inherent in the breed they work with.
Some of the diseases that Sheltie's are susceptible to are: Progressive Retinal Atrophy, Collie Eye Anomaly.  These two eye diseases will cause blindness.  Buyers should search for breeders who test for these diseases.  The breed is also susceptible to heart disease, epilepsy, and von Willibrand's Disease (a bleeding disorder).  Merle puppies with double merle inheritance may be deaf.  Structural problems can include elbow and hock joint subluxation (looseness) and hip dysplasia is possible.
The Sheltie is generally healthy and hardy, generally even-tempered and highly intelligent.  This breed adapts well to apartment and condominium living as long as it receives sufficient mental and physical exercise.  All in all, a terrific family pet, companion, show competitor and/or working dog.