Had to use FR/BA/EZY/MON for positioning to and from an EU based aircraft.
MON invariably are more expensive but not by anywhere near as much as you'd think, it's also the case that later last minute bookings are much more affordable with MON than anybody else.
Never been on a MON flight which wasn't full.
Last minute positioning on FR for €300 into the EU just makes you feel robbed, Didn't end up going on any other website if MON was something near price wise.
Doesn't take much extra per seat to cover the additional costs with MON and I for one would happily pay it.