I got my puppy at 8 weeks and while he is a great puppy, he still has accidents sometimes (my fault, I know) and he can get a bit hyper. I often wonder if maybe he had stayed a little longer, he would be better at those things and not cause the trouble he does now. Of course, it's all part of his personality and I love it about him! I just think if I ever get another puppy, I'll be waiting until they're a little older.
As for car travel though, Helio was absolutely fine when we brought him home (an 8 hour car ride!). He slept nearly the entire time, drank some water out of a container, had a wee in a box we had in the backseat, and didn't whimper once! He had been on three trips to the vet with the breeder, but those were short trips and I was just surprised and pleased with how great of a traveler he was. So I think it depends on the pup!
Mommy to Helio (blenheim male, born 9/23/10)