I have run and rode bikes along this trail and overall it is very nice. The trail is well maintained as far as I have seen with clear pathways and no overgrowth. There are some homeless people's belongings and a tent or two at some places on the trail but that certainly doesn't bother me.
There is usually a good mix of people around which has always kept me from feeling unsafe. One thing that sucks is that during certain times of the year there can be some hoards of gnats right at face level...I've definitely swallowed a bug once or twice.
I usually park at the South Side riverfront park that can be accessed from south 18th street. There is plenty of parking, a few picnic tables, and port-a-potties there.
Very happy to have this trail in Pittsburgh overall.