To run slightly contrary to the thread, when I was living and commuting in Glasgow I found First buses there were generally fine. They weren't angels but I can't recall any truly idiotic and/or murderous shenanigans either. As noted it was the coaches you really had to watch out for.
To run not all all contrary to the thread, I'm just back in from a walk round Chesser/Stenhouse, and I witnessed some ridiculous driving from a First double decker. Along Stenhouse road at the Shell petrol station, the limit is 30mph, though at this time of night you'd hardly think so. The bus was tearing it along towards Calder, engine screaming, the whole thing rocking and bouncing over all the potholes along the bus lane. Had to be at least 40, maybe even 45 or edging towards 50. Bus got flashed by the speed camera along there so it's not just my word for this (if the camera was working....). It's a bad stretch for speeding that, I walk along there most evenings and it's not unusual to see cars getting flashed by the camera, but this is the first time I've ever seen a bus be that determined.
Minutes later a Lothian bus came along at a far more sedate and legal pace, as would be expected, and as I always see.