iPhone sync is always going to be difficult: you are forever chasing a moving target, although the libimobiledevice and libgpod guys have done a great job. The encryption used to sign the music database on devices released after iOS4(+original iPad) hasn't been cracked yet (they changed it from the previous versions), although you can edit a file to tell the device to use the old db encryption if you have jailbroken your phone (I've not tested this with iPhone4S; things may have changed again). Building a product around the feature is probably unwise — Apple is a capricious master.

(Disclaimer: I develop a competing os x music player which can add music to (some) iOS devices)