Contrarian is used to describe a variety of closely related classes of people. In general it's used to refer to somebody who holds an opinion opposite to the consensus.1 This opinion can be political, scientific, grammatical, whatever. In this sense contrarian is a neutral term since the consensus is often wrong. That said, it takes on negative connotations because it's also used to describe argumentativeness for it's own sake either because of reflexive devil's advocacy or just attention seeking.2 This is particularly problematic because the second usage necessarily includes the first. Muddying the waters further, among contrarians the term is likely to be seen as a badge of distinction and understood to mean free thinker in opposition to the dogmatic sheeple. Since the word can carry all of these meanings it is worth taking the time to understand both how it was meant and how it is likely to be heard.

1. Webster
2. Urban Dictionary