j.b can be used in a combo from 6a. I believe the combo is listed as:
6a > j.c > j.b > 9d~5 > 5d~6 > j.c > 5d~6 > j.a > j.c > 9d~6 > j.236bb j.236bbbb.
I've used it a few times in the air, but I found it too slow to be safe, and it's reach isn't that great. As for the tier list, you'll have to get someone with more experience to answer that cause I'm still a newbie.