Several authors apply Information Retrieval (IR) techniques to recover traceability links between software artefacts. Recently, the use of user feedbacks (in terms of classification of retrieval links as correct or false positives) has been proposed to improve the retrieval performances of these techniques. In this paper we present a critical analysis of using feedbacks within an incremental traceability recovery process. In particular, we analyse the trade-off between the improvement of the performances and the link classification effort required to train the IR-based traceability recovery tool. We also present the results achieved in case studies and show that even though the retrieval performances generally improve with the use of feedbacks, IR-based approaches are still far from solving the problem of recovering all correct links with a low classification effort.