I have maybe two playlists (so roughly 1,000 songs) on my iPhone from iTunes. However, I keep my Spotify starred playlists downloaded, and various other playlists and the radio feature for streaming.
I love it. Absolutely worth the money. It's fantastic.
The app itself isn't amazing, but it works and is stable. It's just missing that design flair I love. 😛 I have had it have to redownload the starred playlists before for no apparent reason, but that isn't necessarily a dealbreaker.