I fully support the Elsevier boycott, and came from mathematics (BS and MS) into my current PhD program in an engineering subdiscipline. I'm finishing up my PhD now and starting a post-doc soon.
I've already refused to review an article a few years ago for an Elsevier journal, and the editor seemed quite angry about it.
Once again I have been asked to review an article for an Elsevier journal, only this time the journal editorial board has a prominent researcher who I have had personal contact with, is a recently retired Emeritus Professor from my department, and if he became angry about my refusal he could potentially harm my career.
Does anyone have any specific examples of, or even better statistical data showing that boycotts of Elsevier journals had negative career impacts on early career/junior scientists?