Does it really make that much of a difference? We have a 5+ hr trip next month for a wedding, and DH thinks that it will be better if we change the carseat to be front facing.
What have you found?
He's 16 months and 23lbs (22lbs14oz at the last appt).

Re: turning carseat front facing?
Are you asking whether it makes a difference in terms of safety or the child's contentment with long car rides? As far as safety, I think it definitely makes a difference to keep the carseat rear facing. As far as the child's contentment, I think it depends on the child. DD is totally fine in the car rear facing but I know some children aren't.
Usually on long car rides I sit in the back with her (partly because my brother is usually with us and he sits in the front). I think having someone in the back with the kid seems to help.
Sorry, yes, I know it makes a ton of difference in safety, but DH wants to turn it around for our sanity. I think LO just doesn't like to be strapped down for long periods of time. I would rather keep him rear facing (for safety reasons!), but I'm willing to think about it for this trip.
once you go FF, you won't be able to go RF, at least your baby won't be happy; the recommendation is RF til age 2
yes, I know this, I'm asking how much of a difference it makes in long car rides to keep him happier.
With DD#1 we switched her to front facing shortly after her first birthday and she definitely seemed pretty happy with the new view and it was easier on long car rides to keep her entertained (though she was a good traveler RF). That said, we have not yet switched DD#2 to front facing (she's almost 14 months), even though she's a bigger kid--in our case, I think having her sister in the back with her keeps her pretty happy.
For safety's sake, I think I would support what pp's said and keep him RF and sit in the back with him if you think he's going to get frustrated over the car ride.
Not necessarily true. We switched DS FF for a week when he was 18 months old, then switched back RFing until 20 months and he was fine.
ETA: For us, it didn't make that much of a difference as far as happiness went. We were hoping that it would help with carsickness.
I have two forward-facing little girls (ages 4 and 2) who are a PITA on long car rides, so no, I don't think being forward-facing makes them any happier.
If I were you, I would not forward face the kid for one car trip, because I'd be afraid of two things:
1) we'd get into a carwreck and I'd hate myself forever for not having kiddo RF
2) once kiddo is FF, it may not be easy to turn them RF again
It's funny, it was my DH as well who kept wanting to turn them forward. For some reason he thought it would make this huge difference. It really didn't. But that may just be my kids.
Before going on your trip, be sure to test out the FF on a medium length trip.
We found that DD gets sick (like, puking sick) every time we have her in a FF seat.
I'm not sure what we're going to do when we have to put her FF.
I agree with you that she's probably more grumpy about being strapped in than she is about rear-facing. I think a lot of kids go through a phase of hating the car seat. It sucks, but I think it's a phase that isn't directly related to RF vs. FF.
DS went through a long period last summer (so around 14-16 months) when putting him in his car seat was a struggle every single time. A few times, I even had to resort to calling DH, putting DH on speaker and having DH talk to DS while I muscled DS into the seat. I posted here for suggestions about what to do, and I was even considering switching him to FF if that would help. Several people said it was a phase and said not to switch him to FF just to try to stop the battle. I am really, really glad I listened to that advice. DS now climbs into the car seat voluntarily 90% of the time (and is a little better on long trips), and I feel so much better knowing he's still RF. (We're planning to keep him RF as long as we can in his current car seat, and he has another inch or two of height to go before he's too tall for RF in this seat.)