I have always used Britax, and only Britax, car seats for my child.  He started in a Wizard, moved on to a Marathon and is now in a Boulevard.  It is a huge car seat and probably wouldn't fit well in all vehicles.  I drive a very large SUV and it fits fine, although my older child does say it makes the seats more narrow for other passengers.

I was in a frontal crash last year and it totaled my vehicle.  My child walked away from that accident, as did everyone in the vehicle.  The emergency technicians on scene stated that they had never seen such a large seat and commended me for keeping...