Timely thread, btw. I was on a flight yesterday where some dude had a guitar in a hard case, as his carryon. He was Group 3 so there were plenty of completely open bins. He snagged one pretty much directly over his seat.

This ended up as a completely full flight, and the last few passengers had to check their carryons. No one complained about his guitar taking up space that could have held 3 or 4 rollaboards.