F on IB is OK (and sometimes very nice), it is their Y and J that are awful. IB staff behave quite differently to F passengers and they don't see most of IB's bad behaviour. Plus the F cabin is very small and often empty (more empty than it would be on any of the alternatives). For your routing I would even go so far as to say it is a good choice (but only in F, and only on MIA-MAD).