I think they are mostly bored? The take off and landing can be hard on them (their ears hurt - if they can breastfeed, or have a bottle, or suck on a pacifier, that helps). I think babies are pretty used to being bounced around, one way or another.
The only bit that really sucks, I think, is if they're crawling, because then they want to crawl up and down the aisle, but the flight attendants don't like that much.
I have flown quite a bit with my DS and we flew long haul when he was 6 months old. He was an extremely difficult baby...but was an angel on the flight I have lots of friends who have travelled with babies and pretty much all say it was far easier than they thought. Babies often sleep on planes - something to do with the white noise? DS had no troubles with take off and landing but I did try to make sure he was feeding or sucking his dummy to equalise the pressure in his ears.
We flew to Canada when dd1 was nearly 3 and dd2 was 6 months old. Dd2 was fine, it was dd1 who was bored and after discovering the toilet about 2 hours into the flight kept asking to go just to get out of her seat.
I have flown frequently with my DC's since DS was 5 months old. We regularly travel between UK and Cyprus, a 5 housr flight, so not longhaul but not a short trip either. DS was a dream when he was a baby (he is now 3 and much more difficult to entertain). Make sure you have a few interesting (but not noisy) toys and books to hand, a few snacks to chew/suck, I have never found take off and landing to be an issue with my 2. Have a great trip
He hated the lapbelt thingy during take off and landing and screamed blue murder (sorry but you did ask) and the rest of the time was absolutely fine, a bit bored but nowhere near the nightmare I expected. Take some small toys and books (wouldn't worry about noisy ones, planes are not quiet places - the drone of the engine is deafening I think - and the advantage of longhaul is TV!!
My 9mo was crawling and cruising when I took her to Australia. Fcking nightmare. Sorry. They're too young to understand that they have to sit down on your lap every sodding time the pilot puts the seatbelt sign on for two bumps of turbulance, including if they're asleep in the bassinet, finally, after two hours of trying to soothe them to sleep on your shoulder and you have to lift them out again and wake them to put the stupid pointless lap belt on..
Horrible. I was 6 months pregnant as well, so I suppose that made it worse. And I did the return journey on my own. So I may be biased!
If the holiday is worth the 20 hours or 48 hours or whatever of hell, then fine, do it, otherwise go to France until they're old enough to be entertained by the seat-back television.