This all pertains to Delta.

I hope I never have to fly coach internationally (long-haul) again - I usually am pressed for time and it is a 'night a day difference' being able to sleep laying down versus being in coach and then getting off the plane to go immediately to meetings. When I fly coach, I need a day to recuperate, when I fly business, I don't. I've found that for me and my work expenses, the best of both words is to by an upgradeable ticket with work funds and then upgrade using my miles.

For short-haul international (less than 4 hours), I think 1st class is a bit of a waste as they don't serve you a decent meal usually and the planes on many of these routes aren't yet upgraded so the difference between 1st class and delta comfort was actually negligible for me over the last year...< $100 for an upgrade = sure...>$100 = meh

For domestic flights, delta comfort suffices perfectly, but i love 1st class upgrades. Won't spend a lot of miles or dollars on getting into 1st.

Long and the short of it is that for me, with my current salary, I would never pay outright for 1st/business class, but am always looking for a way to get up there if it isn't too expensive.

One thing that hasn't really been mentioned is the EXPERIENCE of 1st/business. The majority of time I sit in front, I often end up meeting some pretty interesting people and have had opportunities develop from that EXPERIENCE....only thing that happens in coach for me are negative events like getting exposed to TB or having to deal with the horror of a person next to me who should have been forced to buy two tickets because of their weight.