Just finished it myself, you run around as Sarah when some Hiso aliens steal Noctis's phone, but that's about it as far as controlling her as you control Noctis everywhere else and in combat, you do get a Technique for Sarah about midway through though. The end boss was pretty cool, had way more dialog and cutscenes than I thought it'd have especially for something free. It was about 1 hour long. Wish they had the music from it as radio music too, nice Uematsu music.
Also lol at the part at the end where Sarah calls him Noctis and he says "Just Noct is fine" and then she calls him a funny one, I wonder where that could be from ( ͡° ͜ʖ ͡°)