I use my maxi cosi car seat with the bug a lot as I take taxis frequently and use the stroller in town. Once he grows out of the maxi cosi, I'll use the bjorn for taxis, but prefer the safety of the car seat til then.
I lasted about 2 weeks with the car seat on my Peg Perego before deciding it was too heavy and starting with the baby bjorn - I am shocked at myself, I never thought I would do this as back in the UK you would never take a baby in a car without a car seat!