Pros: Excellent SQ, extreme value for money, cheapest DAC/amp with support for iDevices
Cons: Weird form factor for portable gear, some units have a volume control crackling issue, iDevices require CCK which isn't optimal for portability
The iFi Nano iDSD may not be a looker and it may have a small package but it delivers performance you'd expect from regular desktop DACs at an unbelievable entry level price. In addition to that, if you have to think twice about getting a -700 portable amp/DAC that supports iDevices - go for the Nano iDSD and know that it rivals those devices at a price that is 3 times lesser.
This is a transparent DAC that delivers a refined and natural sound that is difficult to find at this price point. Instruments are rendered realistically and vocals are full of texture and nuance. There's just a wholesome-ness and balance to the sound that reminds me of high end British hi-fi. The built-in headphone out can easily drive IEMs with a black background at all volume levels thanks to its potentiometer-less implementation of volume control. It can surprisingly drive LCD2s well enough on its own but what really shines here is the DAC and feeding an amp will be the way to go.
The Nano iDSD was an impulse buy from me - I was at a headphone shop looking for IEMs, spent some time auditioning with the iDSD and ended up taking home both a brand new pair of IEMs and the iDSD. I was that impressed by them and have not regretted my purchase at all since.