This is fine assuming the airside bus is running to schedule (or at all) because if it isn't and you leave it too late you may end up missing your flight! On my last connection through ORD to LHR I was connecting from AA and decided to take the airside bus on the basis (a) I had plenty of time and (b) the landside train was out of service and transferring that way would have required a bus.

There seemed to be only one bus running - either due to staff issues or breakdowns or just a general farce. It definitely didn't seem well organized. We departed T3 having left probably 2/3rds of the connecting passengers behind as there was no room and made it to T5 and the gate literally as they were boarding. Given the wait initially for the bus I doubt very much if those left behind made the flight.