The airside transfer bus only runs at peak times. I wouldn't bank on it...
In addition, the way that US is running this is EXTREMELY DANGEROUS. When the bus is running, customers are being asked to ascend/descend the metal stairs located on the side of the jetway. These stairs are metal grates which are poorly anchored and when several people are on them, shake from side to side. Furthermore, there is no safe way to move baggage up and down between the ramp and the bus when passengers are also there. I am counting down the days until some old lady slips on them and US gets sued. They really need to change this practice..these aren't meant to be used by passengers.