There was a discussion in the AF forum a while ago... IIRC most people said it wasn't worth it.

The difference between Y and J is significant: a crammed seat VS. a comfortable almost-lie-flat with plenty of space, Y food and crap wine VS. J food and nice wine, amenities (noise cancelling headphones, eye shades,...), service, extra baggage. But when you compare J to F the differences are much smaller. The seats are better but not much better. The food is better but I've never placed that much value on this part of the flying experience. (I can always go for a really nice dinner at my destination and save a TON of money if good food is what I want. Besides, plane food is still plane food... it's not like they have a professional kitchen available for the flight.) The drinks are also better (e.g. the expensive champagne in F), but again... I don't think it justifies the price difference (be it cash or miles).

I suppose that if you're considering AF, you should also consider with whom you want to share the cabin: biz travelers (in J) or the captain's family?