It's tough to admit something like that. But, since the confessions are coming out- the only time I did this was a cheat- it was on a domestic 777 (MIA-DFW) where business is sold as coach, and first is obtained with a sticker upgrade from coach. So, I sat in the FC section on a "3-class" a/c, but it was a 2-class service. Only time.

I guess a lot of people here buy F, or buy business (I is pretty commonly-mentioned) and then upgrade into F. For some flights I've looked at recently, it was about $700 or so for coach (then VIP to Bus), vs. ~$2500 for Bus, with the possible VIP to First. For me, anyway, the seat/food differences are not worth the 3-4x price. Of course, sometimes there are really good 'I' deals posted here that may be worth it. This is a definite YMMV.

Cheers.