Pros: Solid bass that's not farty or distorted, very impressive soundstage for a cheap IEM, highs aren't fatiguing at reasonable volumes
Cons: Not much low-level detail.
Context: I primarily listen to music through my setup of FLAC -> HRT Music Streamer II -> Little Dot Mk3 -> Hifiman HE-400. I bought these primarily to use at the gym with an ancient RCA mp3 player that only has 512mb, because expensive gear and sweat don't mix IMO. These setups fill completely different roles, so I obviously don't expect them to compare, but I think it's important to clarify that my reference system costs about 40 times as much as my gym system. Note that my ratings for value, audio quality, design, etc. in the details section are in absolute terms, not relative to the price.
Now, on to the NE-600X. I am extremely pleased with these for the price. I have gone through a number of lower-end IEMs over the past few years (mostly Sony and Altec Lansing UE clones, don't remember the exact models), and these have been my favorite. I have no reservations recommending these given the price, as long as you like bass.
Sonic Balance: Bass-heavy, but not V-shaped. The midrange and treble are balanced and relatively smooth. These are dark and laid-back overall.
Bass: The bass on these is very emphasized. It certainly resembles the typical boomy, basshead bass found in many headphones in this price range, but I find that it's a little better. The bass never gets farty or sounds like it's clipping -- it's just big, wet, and powerful. It obviously doesn't have the extension or speed that I expect in an expensive, full size pair of headphones, but it has a very satisfying sense of weight, pressure, and power. Definition is above average for bassy IEMs in this price range, but nothing fantastic. It is slow in absolute terms, but reproduces heavy textures well. Poorly mastered rap (i.e. most popular rap) sounds more fun on these than on my $400 planar cans, because these put enough oomph in the midbass to mask the lack of subbass in the recording.
Midrange: Not much to say here. It's fine. The bass can bleed into the midrange at times, but the NE-600X is better in this respect than other IEMs I've had. Midrange detail is above average at this price level.
Treble: The treble is smooth and not fatiguing. I don't hear any sibilance or harshness at moderate volumes, but there is not much air either. I appreciate that the treble isn't boosted for the sake of artificial detail, but it is on the dark side. The treble is the first to deteriorate when I crank it however -- it starts getting splashy, uncontrolled, and a little harsh.
Soundstage: I was most surprised here. Obviously the soundstage isn't big, but it is very natural for a cheap IEM. There is layering and separation between different instruments, so that I have a sense of how different instruments are located in the room. Vocals are clearly presented in space, and samples and other effects are positioned where I expect them to be (after hearing the same song on my reference set up).
Detail: These don't reproduce much, if any, low-level detail, but textures and tones are resolved in a natural way.
Build: These are very solid. They are all plastic, but not flimsy in the least. A little more heavy than other IEMs, but not enough to be annoying. The cable connections to the earpieces and to the jack feel very secure, and the jack is at a right angle. I also appreciate the flat cable, as it resists tangling effectively. The cable doesn't feel like it could ever snap in two, unlike some of the delicate cables I've seen on Sony IEMs.
Fit: This is very personal, so I won't complain too much, but I have a hard time getting a seal and they tend to fall out more easily than other IEMs. This is probably due to my ear shape, but the insertion is very shallow, which doesn't help. I am usually aware of them in my ears, so they don't disappear completely. However, they never hurt.
Isolation: The insertion is shallow as I said, so these don't completely cut out the outside world. However, they do block enough noise that I don't hear normal low-level background noise when the music is off, and probably wouldn't notice someone trying to talk to me if music were playing. They aren't going to *completely* isolate you from loud background noises like traffic on a city street, however.