Oh, I also would LOVE to know sometimes what other people think of me because of the stuff I carry with me (I tend to believe it is: "What is she training for?", because I always carry A LOT of heavy books while trying to be REALLY fast).
However, I believe, we should reframe the question "which impression do we make" a little, employing some really interesting stuff . I'm sure you know scientific approaches as cybernetics, which advocate a model called "the black box", meaning that you only know input and output of a system, and not what is going on inside. This model has also been applied to communication, thus (oversimplified here) making people's minds "black boxes". So, generally speaking, not everyone will have the same notion about you using a fountain pen. Some might not even realise you are using one. But you can't know. Therefore I think it is a good idea if we do not ask "Which impression do I make?" but "Which reactions do I get?". Pay attention to the output, people's behaviour. Do you notice any changes in the way they interact with you (maybe applying the great lessons in body language we are getting here)? (And don't forget that you are a black box, too - factor in that you also have preconceived notions about others that will influence how you judge their reaction )
So the bad news is: No one universal answer and no access to impression others have of us. But the good news is: loads of individual cases to explore and plenty of opportunity to study body language