I've used 18GB. I think I have 6 or 7 movies on it and 1,100 songs. Also 500 photos and 30 podcasts and videos. Definitely more than enough room for me. I just throw movies on to fill up space.
I only used 1.7GB so far. I just got it as a replacement for my 3G though and haven't got around to putting all my music and putting converted movies on it. That will take up at least 20GB.
Almost 14gb, but I have another few gb of music I am planning on putting on today or tomorrow, along with a couple of audiobooks, so I should be getting close to 20 shortly. I <3 my iPhone so much.