I have used Elixir Nano 80/20 medium gauge strings on my F50r for at least the last year. I like them for that guitar for reasons mentioned by others.
Most notably, for me, they last forever. I can get a solid 5-6 months out of them when playing that guitar 3-4 times a week, a couple hours at a time. They settle in after a couple days, don't stretch much or lose their tone and brightness for many months. I don't have any problems with shedding or string breakage. They are not the most complex or interesting string out there, but I find them simple, true and clean sounding which works well for me on the big rosewood jumbo. It has plenty of richness and body in the tone with any string, so I like the clean, bright tone signature of the Elixirs plus the added longevity.
I like Pearse 80/20 as well. They have richer harmonics (IMHO), but they only last about a month before becoming dead. Elixirs cost twice as much but last me 5-6 times as long.
I am currently auditioning some strings on the newer D-40BJ. Haven't found its perfect mate just yet.
Michael