"They" as a gender-neutral singular has been in use for at least five hundred years. I think it's perfectly good and is much better than clumsy constructions such as "he/she," "he or she" or the laughable "s/he." Quite often I have to reply to a letter in which the writer hasn't given away their gender, usually some official or other. I hate writing "Dear Sir/Madam" or similar horrors, so I occasionally resort to things like "Dear Customer Services Manager" (if in doubt, always promote them).