Only have good experience. 3 years in with our 6 year old GSD rescue.
I think a lot depends on the rescue too. We went to German Shepherd Rescue Scotland, have only got good things to say about them. They don’t have kennels and you don’t get to pick as such, the dog is matched to you (you can specify what you are looking for, male/female, age, good with cats etc etc).
It was quite a thorough application/interview process. We were interviewed at home, had to rebuilt some fences (judged to be not high enough). After a month with our dog, we were revisited to see how she was settling in.
The assessment of ours was bang on, good with dogs, good with children, good on lead, not good with cats etc etc. We got to miss out the toilet training and mouthing stages! We couldn’t have got a better dog :)