X3 plays high res, meaning 24-bit 96khz recordings. I'll leave it up to the thread in the sound science forum for you to read and make a decision whether you'd want to try such files, but even for regular 16-bit 44.1khz files, there area lot of improvements. I have an S3 and played around with an X3 at a hi-fi show last November, so these impressions I have come from about an hour of listening back and forth between both driving my Aurisonics ASG-1.
1. X3 drives my IEMs a lot better. This is very surprising actually, since I assumed that a very efficient IEM presenting a not-stressfully low (to an integrated audio chip, like that on a smartphone) 32ohm load would not improve much, but it does. Dynamic range is noticeably better, with snappier attack and better-controlled decay of the percussion. I can go louder with the X3 but note that I can't really find the limit, nor care to do so, given the S3 can drive it loud without too much distortion than my ears would care to handle. Also I question why the distributor strapped them to some portable amps, when even the AKG Q701 was driven well enough by the X3 on its own.
2. Background noise. The S3's very, very low hiss (on this 32oh load) actually sounds loud by comparison thanks to the noticeably blacker background on the X3. Not even switching to Airplane Mode on the phone can isolate the noise, which means that isn't the problem.
3. Better fullrange response. On the S3 the sound is quite dark; if I run a sweep from 20hz to 20khz on it, with the ASG-1 it drops off after 12khz, then disappears at 16khz. The drop is less severe on the X3 (although I'd have to really pump it up to still hear anything above 16khz), and better yet on actual music the treble noticeably opens up. There was more detail to the cymbals, more detail in female vocals, sopranos sound more natural, etc. (Note: an IEM wthat won't drop off as severely as the ASG-1 will not be that bad on a smartphone in this regard; like the Westone 2 for example)
Still, I went home without spending on the X3 for these reasons:
1) Right now $200 isn't exactly just disposable for me
2) I got a subsidized phone, and along with #1 and the efficiency of the ASG-1, I can live with the smartphone for a while
3) After using iPod wheels and now touchscreens, I didn't enjoy using buttons; that said I didn't find the UI software to be the problem - for me it really was just the input hardware.
4) I might usually have a laptop bag or camera bag with me, but I held out for smaller DAPs like the upcoming X1 to go in my other pocket (portable charger goes in the bag with my IEM Otterbox case), considering the X1's output power won't be overkill for my IEMs and I already had a reference amp at home for my HD600 anyway (which uses the Android phone as a USB audio server with a convenient stand, see below).
Basically, if you don't have the same reasons for not getting that I do, then consider the pros in the first list.
Thanks a lot for the comprehensive comment, it is very helpful