Yes, we flew far longer flights (SYD-United States, as well as PHL-Europe). Obviously she didn’t spend the entire time in the car seat, but the long hauls where we had it were nicer than the ones where we didn’t. The airline-provided bassinet was annoying, partly because it was situated where lights distracted her and partly because at least on some airlines she had to come out when the seat belt sign went on. (I don’t remember if that was AA policy; I recall that either US airlines required kid out of bassinet and foreign airlines didn’t or vice versa; we flew long hauls on QF, DL, US/AA, VA [Virgin Australia], and BA, all in coach.)

18 months is a much more difficult age to fly with than 3-6 months because they require far more attention and of course are much bigger to hold in the lap. It is virtually certain that an 18-month-old will be in your lap for part of the flight whether or not you have a car seat on board. But it’s totally manageable and worth it!