I worked 6 months med surg before going into psych which was and still is my first love. Did those 6 months help? Not sure, it was so long ago!
I guess I feel like any experience is valuable, but would I have been a good psych nurse without it, sure. It's up to us as professionals to stay on top of changes in nursing. Things change so quickly that meds, for example are completely different from what I used years ago. That's true for medical, surgical, cardiac, AND psych. WE have to take responsibility for our practice, and that might mean looking up the newest cardiac med our patient is taking, just like a med/surg nurse may need to look up the new antidepressant a surgical patient is taking.
I guess what I am saying is if this is what you want, go for it. I agree with Maureen, of course this is "real" nursing. I have cared for people with schizophrenia AND diabetes, depression AND heart disease, and a couple of women who were bipolar AND pregnant, as well as patients who have only a psychiatric disorder. You will see an incredible variety of patients who need your skills and education. Good luck, if it is what you want you will love it.