As an aeroplan member, on routes that are served by AC with one flight per day (YYZ-SCL, YYZ-LIM, YVR-SYD), I have tremendous difficulty finding any business class availability at the lowest fare redemptions. I often find there is no availability on flights 330 days out.
However, if I wanted to redeem for a destination with lots of frequency (YVR-LAX-OGG) then it is no issue on UA flights.
If J class is important to you for long haul, I'd look somewhere else.