Ditto on the previous info provided about allergies. Check it out with your allergist BEFORE you buy one. Don't assume a 'hypo-allergenic' dog won't affect you.
Are you buying from a breeder? If so, you may want to ask some questions about the dog's traits and its parents and sibs, etc.
"The typical fine, straight, and silky Yorkshire Terrier coat has also been listed by many popular dog information websites as being hypoallergenic. All dogs shed, and it is the dog's dander and saliva that trigger most allergic reactions.
Allergists do recognize that at times a particular allergy patient will be able to tolerate a particular dog, but they agree that "the luck of the few with their pets cannot be stretched to fit all allergic people and entire breeds of dogs."
The Yorkshire Terrier coat is said to fall out only when brushed or broken, or just said to not shed. Although neither of those statements agree with what biologists, veterinarians, and allergists know about dog fur, allergists "think there really are differences in protein production between dogs that may help one patient and not another", meaning that some allergic people may not have allergic reactions to a specific dog, like the Yorkie."
My expereince w/ Yorkies
I do know a little bit about Yorkies. They've a decent playful disposition but like any small dog, they can be prone to 'small dog syndrome'. They respond well to calmly assertive personality. Unless you're well schooled, you can get more than you bargain for. Though they're far better than some small dogs (infamous chihuahuas or mini dachshunds..weiner dogs).
Check out National Geographic channel TV show called 'Dog Whisperer'. Cesar Millan rehabs dogs and trains people. He teaches people some awesome techniques to keep your small dog from taking over your home and your world. Here's a link to his bio" http://en.wikipedia.org/wiki/Cesar_Milan
Some Yorky Breed traits
The typical life span of a healthy Yorkie is 10-15 years.
As a breed, some Yorky dogs are predisposed to have collapsed tracheas. Tracheal collapse is a progressive weakening of the walls of the trachea, occurs in many toy breeds, especially very tiny Yorkies It's wise to avoid the tiniest of this breed.
I've a friend who owns a pure-bred Yorky but sadly this one has a collapsed trachea. It's heart breaking as it impairs the dog's breathing and eventually will shorten his life. Another friend has a Yorky who has 'slipping knee-cap problems.
Don't be alarmed about all the Yorky trait info as thos problems may not be a problem for your Yorky. It's good info to know and ask the breeder about. I hope this helps you out with your decision.