I've known a few well bred GSDs that I've loved and a lot of not so well bred ones, some very sweet and mellow and some with nasty behavioral issues.
Lines vary a ton from pet to show bred to working and from euro and American lines. You could easily meet a dozen breeders who's dogs you can't stand and then hit one you love.
All in all - fairly drivey but not in the same way as a BC or even a Malinois IMO. Prone to anxiety, sometimes SSA, general DA seems more rare. Not many are going to be "dog park" dogs but most will have minimal reactivity if properly trained and socialized and can live with other animals. Some can't live easily with small animals and some can, that seems pretty hit or miss. Usually good with kids if you discount the unfortunately large unstable or mishandled population. A well bred or well raised GSD should be great with kids and even enjoy them.
Can be protective, territorial and are really in tune with their owners so without heavy socializing, if something makes you anxious, they might react. I kind of tend to not recommend them as much for owners with mental health issues, panic attacks and the like, or as psychiatric service dogs, because those situations can be stressful for them.
They are active dogs and need as much exercise as other working and herding breeds, but I think the requirements are something a typical active person can provide, whereas breeds like malinois and BCs and such I kind of think should be reserved for those extremely active families. The exercise needs seem pretty on par with an active lab or boxer or Aussie. This obviously depends on the line as well.
Lots of members here have them, so hopefully you'll get more in depth responses! My favorite GSDs have been police department wash outs and one German import. One of the wash outs was a working farm dog that ran coyotes off an Arizona property. They're a breed I'd love to own one day.
ETA: as with any popular breed, health isnt great. Prone to hip and I believe sometimes elbow dysplasia, eye problems, epilepsy, degenerative myolepathy. (I have no idea if I spelt that right). Good health testing is a must. I'm not sure on standard sizes but there's a huge range. Some females are as small as 45-50 lbs . Sometimes I see huge males and people bragging about their 100 lb shepherds, but even 85-90 lbs is a pretty big dog IMO. They're certainly around, though.