Do you want to sync the library between between Apple devices only, or are any other devices involved?
If you only need syncing between computers with iTunes, and iOS devices,
iTunes Match could be the solution. It's $24.99 a year, which I think is reasonable compared to, for example, a large iTunes library on Dropbox.
It has some limitations and quirks though. There's a 25,000 track limit, which is surprisingly easy to hit. The limit does not include songs purchased from iTunes, just the ones you upload yourself, so depending how large your music collection is, and how you have acquired it, this might or might not be an issue.
Occasionally syncing can be a bit wonky when uploading large amounts of songs at a time, and streaming playback on mobile devices seems to require a good connection. In many situations I've been able to use Spotify with high quality enabled, but iTunes streaming has been unusable. But you can also download the tracks instead of streaming.
The streaming is a nice touch though. I don't need to fill my laptop HDD or phone storage with the library, but I still have access to it anywhere.