Their owners often spend small fortunes just keeping their pets alive from year to year. Because of poor health and genetics, it is very rare for these pets to live long. You will be able to see how the puppy was raised, under what conditions, and you should be able to see at least one parent. From a pet store, you receive none of this. Puppies from a reputable breeder have been well-socialized in a variety of settings.
Solid temperament is very important to the reputable breeder.
This is not to say all breeders are utopia. Where-ever you decide to patronize, check it out carefully. If you're not comfortable, take your business elsewhere. No matter where you buy, be sure to get, read and understand the guarantee. If the seller makes a verbal promise that isn't in the contract of sale, have them put it in writing.