I flew with my Britax Advocate 70 (Same seat as the Boulevard but has the SICT cushions on the side so a bit wider.
I had no problems with it. I installed it RF on the plane both trips for my 2 year old son. Did not need a seatbelt extender. We put him next to a window. Bonus to RFing on a plane? They can't kick the persons seat in front of you.
We flew Southwest Airlines, if that matters.
We carried the carseat through the airport... I was traveling with 2 beefy guys (husband and his friend) and they just took turns carrying the seat.
But if I was to travel again, I would buy the rolly cart to strap a carseat onto and use it sort of as a stroller.
[ame="http://www.amazon.com/Britax-Seat-Travel-Cart-Black/dp/B005CNEQK0/ref=sr_1_1?ie=UTF8&qid=1367121620&sr=8-1&keywords=britax+travel+cart"]Amazon.com: Britax Car Seat Travel Cart, Black: Baby[/ame]
We did have to lift the carseat up over the aisles as we carried it onboard the plane. That didn't bug me.
I agree, it was nice not worrying about compatibility issues with the rental car. It's easy to install a Britax into pretty much anything. Not all car seats have that luxury.
I agree its worth dealing with a heavier carseat to not have that concern of potential incompatibility in a rental car.