Pros: transparency, soundstage, price, build quality,
Cons: USB noise, slightly bright
Build: Simply put it, the build quality of the Modi is 100% perfect. Made in the USA and it feels and weighs more than you would think for something so small and only $99.
Sound: The sound of the Modi is pretty transparent, it doesn't alter the frequency response very heavily. Using the Modi improves the technical areas of my headphones and speakers, like the detail and soundstage. One of the most noticeable difference versus onboard sound is the increased soundstage. Everything sounds a lot wider with my HD 600. A good example of this is the first minute of Avenged Sevenfold's "Victim". Everything sounds less mashed up and more separated, the guitar on the right also moves farther towards your right instead of being mixed up in the middle. The Modi also slightly brightens the treble with most headphones and speakers I've paired it with, this was good with my HD 600, cymbals stood out more and you get less of a laid back tonality. The slight treble boost is very subtle however, it won't make any headphone you pair with it instantly become bright.
The only bad thing I can say about the Modi is that because of the USB DAC input, it suffers from USB noise on some computers. Using Modi with my PC, it doesn't have a fully black background after 50% volume and the low end of the bass makes a static sound and it plays high frequency tones when playing 30/40/50 hz sine waves. Using a USB isolator would eliminate all of these problems, but an isolator is about $50. Additionally, plugging the Modi into my iMac or my laptop, the background becomes black even after 50% volume and I hear no problems with static and the bass. The high frequency sound being played with the bass is rarely noticeable in 99% of my music (and I listen to a-lot of dubstep and bassy songs) but it is there sometimes and when it happens it is very annoying.
Overall, it is a great DAC assuming you don't get problems with USB noise. It improves the sound if you're running a computer with the integrated onboard sound. The great thing about Schiit is you have a 15 day money back guarantee, so if you do have any problems you can return it.