I go First if there is an upgrade available and it's not too expensive. When I was working I usually knew about journeys well ahead of time, so could take advantage of any offers, and upgraded regularly. The experience is variable. Sometimes the extra spend is pretty much covered by what you would pay for coffee and food, and on a long journey it's good to have quiet and a bit more room. One journey from (if memory serves) Crewe to Penrith was awful, as the FC carriage was next to the buffet car and the door was broken, so every time someone came through (which happened regularly and disturbed the 'peace') it stayed open, and the two of us in the carriage had to take turns to pull it shut to keep out the draught. I would have been better off in Standard and resented having paid more for the trip.
Another trip from Newcastle to Kings Cross was rescued by FC, however, as there was a problem with the heating. It was red hot and the rest of the train was crammed, as the train before had also overheated and the passengers decanted onto our one. It was full in FC, but not uncomfortably so, and we were given fresh chilled water at regular intervals. I think the people in Standard had to buy theirs when the trolley came round, which would be rarely, as there were people standing and it would struggle to get through. I was so pleased not to have to stand for hours in an overheated crowded train.
My favourite memory of FC was travelling from Glasgow in terrible weather. It was near Christmas, and the snow was deep. The trains after mine were cancelled, and ours was very busy, as other trains were already cancelled, connecting trains had been delayed and people were desperate to get home. A guard came in and asked if the people in FC would mind other passengers sharing, and of course we agreed. A group of students came in and were clearly not used to FC travel, as they kept commenting on it and politely asking people if we minded their sitting with us. When the refreshments arrived they weren't offered any, so everyone who had a FC ticket took two or three pastries and drinks, and gave them to the students. It was lovely to see their faces - they couldn't have been more pleased if it had been a banquet
. They had waited for ages to get a train to take them home for Christmas, and thought they'd have to have to stand for hours, so a comfortable seat and free Danish pastries was an unexpected treat.
These days I don't used trains as often as when I was working, and whether I upgrade depends on the length of the journey and how much it will cost - fares are high enough as it is. If I'm travelling at weekends I always upgrade though, as being stuck in a carriage with Hens or Stags can be a nightmare, but on a Tuesday morning or similar I only do it if there is a negligible difference in the price.