Just got back from a trip with my baby, and ...
Well, we also have the Britax, far too big to travel with if you ask me (along with our small stroller, heavy overstuffed diaper-bag-cum-carry-on, video camera bag, small-size guitar, Ergo, and of course our DS - and all the luggage we'd checked, three large bags ... no way!). So we went to Target and bought a smaller, cheaper carseat, a Costco. $50.
OK, it's rated 5th by Consumer Reports as opposed to 1st, as the Britax is (in the convertible category). But I loooked at its detailed rating and it does not size up too badly at all, really. It's just not the best. But the best is the size of a VW bug. I even splurged and bought the tote for it (at One Step Ahead), not just for the convenient carry handle but also because I hate the idea of something like airplane grease getting on the surface my son sits on.
Long story short, we didn't take it along after all. At our destination we would need it for three taxi rides tops. Shortly before the trip, my SIL (who was traveling with us with her 7-month-old DS2 and 3-year-old DS1) reported that her pediatrician "gave her the green light" to leave carseats at home and settle for 1) the portable 5-point Tote 'n Go for DS1 (linked above), and 2) wearing DS2 in the Baby Bjorn during the car rides. We were skeptical and worried and totally waffled about our decision. Then we decided to risk the rides without carseat, too. I wore my DS in the Ergo in the taxi/airport limousine, sweating bullets the whole time and feeling pretty guilty for choosing convenience over potential safety. (We also ended up taking a few taxi rides in Mexico, which would have been scary rides even with seatbelts - which there weren't any. Thank god for the Ergo in those cases.)
Anyway. Verdict. I think I'll take the carseat next time around, even if we again don't buy an airplane seat for DS. A point made on another thread about babies and travel (posted in Life With A Babe) was that you can never be sure that rental/shuttle-supplied carseats are as safe as your own. They may be older and thus expired (don't they "expire" after 3-5 years?). The materials may have succumbed to general wear and tear. They may be recalled models (could you recognize a recall on the fly?). They may even have been in an accident and thus be no longer dependable at all.
What a pain to tote a carseat along for just a few rides, especially if you don't purchase a plane ticket for your baby, but at least it can be checked (even curb-checked). In the tote, it's really just a small, light piece of luggage. I guess if I were traveling to a city, I would eschew taxis and take as much public transportation as possible rather than lug the carseat all over creation. Sightseeting with a carseat in tow? Museum-going with a carseat in tow? I don't think so.
Re the Tote n Go: My SIL loved it. It was just great for her 3-year-old. Extremely portable and also very easy to install.