The first thing you should know about GSDs is that, even more than many other breeds, research, research, research is absolutely paramount.
Do you want American lines, or German? Working lines or show? East German, DDR, or Czech? Do you want a dog for conformation, schutzhund, tracking, herding, obedience, agility? What do you know about their health issues?
Black and tan, saddle, blanket, b-color, sable, solid black, white? Longhair or standard coat?
The German Shepherd Dog is probably the most loyal breed of dog there is. He will breathe, live, and die for you as long as you respect him and treat him as an "equal". This of course does not mean you allow the dog to run your household, but if you do not respect the dog, he will have no trouble saying "Eff you, you jerk!" and leave you stone cold alone.
The physical exercise requirements actually aren't as demanding as one would think. What gets most GSD novices is the MENTAL requirements. This breed will generally misbehave because it's bored, not because it's underexercised (though that can be the case as well). A mile or two jog or bike/jog during the day combined with some good fetch is satisfactory physical exercise for them, but they absolutely thrive on puzzles and mental challenges.
This is a breed that loves to learn, and they are incredibly easy to shape and teach because of their willingness to please. Highly biddable and multifunctional, there is no better breed, in my opinion.
Great with children, especially if raised with them. Naturally wary of strangers, one does not need to panic when their Shepherd meets a new person, but be aware of their temperament, and explain to those who wish to pet that when the dog shows disinterest, he is not unfriendly, he just simply does not care. If the stranger is not their human and they are not a threat, the dog feels no reason to show interest. The GSD is supposed to be an aloof animal, and any dog that is too friendly (Strauss), too fearful (Zander), or too aggressive (A GSD at class I know), is faulted in temperament. However, I will take the dog that is too friendly over the last two.
Leadership is extremely important. You must be firm, but fair. Do not waiver when those brown eyes stare into yours and say "Puhleeeeeeze don't put me in the kennel!" Do not falter when your dog shrieks like a gutted banshee to be let OUT of said kennel. Do not second guess when your dog has a spazz attack and needs to be convinced that the kitchen broom will not consume him.
Socialization is also important, and because of the natural temperament of the breed is to be indifferent, there is no such thing as too much socialization. You should expose them to anything and everything (even though you will run into things you NEVER thought of....my Strauss is afraid of people in costumes if he can't see the face).
Understand that while beautiful, loyal, and true to their hearts, the German Shepherd Dog is not a healthy breed. I hate to admit it, but it's true. Some of the things the GSD is susceptible to are:
Hip Dysplasia, elbow dysplasia, megaesophagus, bloat, torsion, pannus, dengenerative myleopathy, spondylosis
Screening the first two is simple...you just go to a breeder that OFA's and whose lines have a strong OFA background. Bloat and Torsion is somewhat thought to be genetic, but, there are health issues in every line. This is a deepchested large breed, so they are naturally more prone to bloat/torsion, and is just part of the package. DM is something that should be read up on and studied, as well as spondylosis.
They can be highly prey driven, no matter what the line. While they can be taught not to eat cats that YOU own, if there are outdoor cats on your property, the "Kitty Courtesy" may not extend to them, and it is something to be aware of.
This is a wonderful breed, but not one for the faint of heart or feeble of mind.