Depends on regional context. I don't know about the rest of the country, but I learnt from experience that in Central Java aku, in spoken Indonesian, still retains its Javanese nuances for many if not most Javanese and thus should be avoided in favour of saya when addressing someone to whom you would not use 'aku' if speaking Javanese. So in that respect, it is not interchangeable with saya, not in Central Java anyway. In pure, non-regionalised bahasa baku, aku seems most prevalent in poetry and popular lyrics. http://www.ausdag.blogspot.com.au/search/label/Aku%20dan%20Saya

DavidG