For iTunes, Google Music provided a pretty darn seamless transition. You download their client software, it scans your iTunes library, and registers the songs you have with Google Play. Songs it doesn't recognize, it uploads.

This is a double-feature because Google Music is both platform-agnostic, and integrates with Android.

Google Music itself is not necessarily the most amazing user experience, and someone with a bajillion music files may be upset with it, but for me the high quality (all purchased & "recognized" music is 320kbps) and general platform- and client-agnostic nature trumps music management "power".