Transfers or connections between lines are a key element of public transport systems. Yet this aspect is not always sufficiently reflected upon, neither on the side of transport infrastructure designers nor on the side of its users. Related to a vast number of variables of multiple kinds, the literature has so far been unable to draw clear and consistent conclusions about the role that each element plays in users' perceptions. This study seeks to identify the factors that are most important for Paris Metro users with regard to transfers. To this end, a survey was conducted based on a real-case trip with 4 alternatives. Two semi-directed interviews were also conducted with two of the survey participants. The study concludes that the most relevant factor for Paris Metro users when it comes to transferring between lines is the time they have to walk between the arrival and boarding platforms. In addition, it identifies several key facts such as that: (i) the vast majority of passengers prefer to make a connection if it allows them to save travel time; (ii) the number of transfers plays a major role; (iii) there is an underlying concern about security conditions in stations; and (iv) a high percentage of users choose a priori less favorable transfer options simply to avoid passing through a particular station.