I read through some previous posts and found multiple opinions. After wading through them all I came up with these facts:
1. Airlines will allow you to hold an infant on your lap during the flight.
2. In France it is legal to hold an infant on your lap in a taxi.
2. Most car seats (such as the Britax Marathon which we have) will be secured with just the regular lap belt on both planes and taxis.
Those are the facts -- and the opinions are:
1. I'd be an idiot to spend the extra $$ on a seat for an infant and the extra energy and time carrying a carseat around France.
2. I'd be a terrible mother not to.
As you can see -- we're torn. If I got the facts wrong, please let me know. I'm pretty sure those opinions accurately portray the two sides of this fence. But I do have a couple questions that weren't answered in previous posts that I could find. If I do lug my gigantic car seat around Paris and want to strap it into every taxi, will the cab drivers even stop for me or take one look at us and drive past the hassle? If I do get a cabbie to stop and he takes us to our destination, what do we do with the car seat? I could see that at a museum we might be able to check it, but what about restaurants, or what about if we just want to walk around another neighborhood? For those of you who would like to weigh in, my baby will be about 6 months at the time of our trip. We're most looking forward to the museums, the restaurants, and just walking around the city -- no night clubs for us. Thank you all for your advice!

Well, a car seat might be the least of your problems with a 6 month old in Paris, but maybe you could just take the subway?

We have travelled with the bulky carseat on many occasions. My advice is to take the carseat on the plane and in the cabs. When we have travelled with carseats in the past, we usually only used them to and from the airport and on the plane. Generally we use public transit or walk in foreign cities. We had a carseat bag that was wonderful. It was a huge backpack that we stowed the carseat in to carry. We also loved a baby backpack when our kids were young. It allowed us to better navigate crowds and museums, although an umbrella stroller works too. And yes, we had to pass on some cabs because they did not have seatbelts in the back seats( I think that was in Lisbon). But, we just got the nest cab. Paris with kids is different any way you look at it. I would take the carseat and be safe. On a side note, we were required to take a booster seat for our kids to France,because of their strict rules.

Happy planning,
Travelatte

Of the month or so total I've spent in Paris during the last few years, I can count on one hand the number of times I've taken a taxi in Paris. I can honestly only remember 3 times. One time from CDG airport and one time to it. Another time from the 12th arrondissement into the 1st late at night.

There's a reason I've almost never taken a taxi in Paris. Paris taxis do not normally stop and pick you up off the street. They wait at taxi stands. By the time I look around for a taxi stand I've invariable walked by a Metro entrance.

Why don't you plan your trip around not taking taxis? Use the Metro in town with a super small umbrella style stroller. This is the kind that folds up almost as small as a large golf umbrella.

Lots of luggage? Divide and conquer the airport. One adult with the baby on the RER & Metro into Paris. The other adult catches a cab with all the bags. You can race and see who gets to the hotel first.

Hi BK,

For the small amount of time that you will be in a cab, the probability of injury to your child is miniscule.

If the plane goes down, a car seat won't help.

My personal opinion: take the seat, not simply for the safety concern, but -- equally important! - to give you and your significant other a break on the long flight over (I'm assuming you are traveling from the US/Canada). We took 2 trips involving 5+ hour flights when my daughter was between 6-8 months old. She was quite active and teething at the time to boot. I personally would've gone nuts if I had not been able to put her in her seat with some toys at various times during the flights. This also allowed her to nap much easier. She hasn't flown to Europe yet but I would argue you need the seat even more on a longer flight.

I don't know what airline you're traveling or where you are flying from but I can't imagine holding a baby for 9 hours straight in those teeny seats on British Air (that's my flight from Seattle and the airline I usually use). Doesn't seem fair to either of you.