I've compared both the ODAC and the Dragonfly to my CEntrance DACport LX.
Considering only the sound quality, ignoring size and convenience factors...
if you already have an amp, the Dragonfly offers nothing over the ODAC, in my opinion - especially not for more than twice the price. The ODAC and the Dragonfly sound very similar to me, overall, with the ODAC actually offering a bit more detail and transparency.
If you don't have an amp, keep in mind that the Dragonfly's amp cannot drive fullsize headphones with authority, even though it works well with IEMs. Sure, it can be loud, but it can't deliver the dynamics and bass control of a portable headphone amp like those offered by Meier Audio.