As far as I can tell, it is STILL the only one that lets you modify playlists on the fly. I don't know why others can't figure out how to append or insert songs into the current playlist. I swear, I used that function all the time when I wanted to hear a specific song.
Google's Play Android application certainly let's you insert playlists/songs/artists in to the current running order... I think PowerAmp too. Took them a while (what 10+ years) to catch up though.
It's been a while since I used my empeg, but I'm pretty sure it didn't let you modify actual playlists - just the running order.