When it comes to smartphones, X1 sounds better than any one I owned (older Droid Motorola stuff as well as Samsung Note 2, S4, and S5). Also, if I want to enjoy my music, I don't want to be interrupted by emails, texts, updates, or any other notifications that run in the background and make the playback choppy. Furthermore, you have a lot of electronics cramped into smartphone shell without any proper layout partitioning of the ground or room for filtering caps, thus a poor signal to noise ratio. I know, some EU or Chinese models now use a high quality DACs, but US versions have water down hardware or if you are on a carrier like Verizon - you can't use any of the Chinese phones that require GSM sim (Verizon is 4LTE CDMA). 'Nough said about smartphone.
Regarding comparison to other DAPs, X1 is not designed to be the BEST DAP among everything else available out there. It's the best entry level budget DAP in $100-$150 price range. If you have unlimited budget or your budget is in $300-$500 range, there are much better options. But in a lower tier price range, nothing beats X1 (sound quality, build quality with metal casing, scrolling wheel control, powerful DAC, HO and LO selected outputs, file format support, EQ, microSD card capacity, etc). You can find plenty of mp3 players that can play music. I just posted a review of $20 ONN mp3 player that doesn't sound too bad. But if you want a real deal DAP and have $100 budget - there is nothing really to sell here because I don't believe there is anything else available in that price range that can beat X1 price/performance ratio.