This is a late reply, I'm sorry.
As to quality, I think you have qualify what quality means. If it means being closest to the actual waveform and reference level, then no, the GoDAP isn't even close. But, it adds more power and has a pleasant roll off like a NOS DAC that makes certain sharp headphones/speakers sound more mellow, laid back. As a reference, it really does fall flat, but I can't imagine that is what Venturecraft were going for. This DAC/amp is pleasant, not perfect.
I'm sorry I'm late with this.
The iPhone has a pretty damn good output system, but it is reference aimed, meaning it will be sharp and as flat as possible. If you have a newer iPhone, like iPhone 4S, you will find that it has enough headroom to drive low Ω balance armature phones perfectly. But, it will retain that pristine reference sound. The GoDAP honestly doesn't retain resolution as well as the iPhone does, but it is warmer. Some might call it more involved.
It has a quality all its own, but it isn't the quality defined as 'better'.
AnakChan & shigzeo, do you guys think that the audio quality is enough of an improvement over the built-in headphone out of the iPhone 4 to warrant a purchase for use as an amp only? In other words, is it worth it if I never plan to use the optical out? My headphones and IEMs are pretty easy to drive, and I only rarely plug my iPhone into the stereo.