The RE252 has excellent timbre in classical music, and a good rawness in alternative rock, the overall sound-quality is good and even (with a slight mid-focus).
It loses out drastically to the EX600 in soundspace, and loses out to the RE272 in tonality and pitch, + quality of the highs.
The RE252 has slightly more bass impact than the RE272 (if you can get a good fit with the special shape) but the bass quality of the RE272 is better.
My overall rank would be RE0 --------> RE252 ---------> RE272 in sound quality, however the RE0 and RE252 have some nice qualities of their own, if you value an easy-listening IEM with a slightly warm tone and even frequency response the RE0 is pretty good, if you value a tight fit (possibly) rawness of note, excellent timbre and slight mid-focus with a closed-in soundspace (sometimes feels more intimate) then the RE252 can be very enjoyable.
The sound-quality of the RE272 surpasses both, and it becomes an even contender with the EX600, where the RE272 has better tonality and a 'delicate touch' and power behind all the notes, it's like iron with talcum powder.
The primary lacking of the RE272 is in the bass, which is quite faint but very good quality, if you're using to heavy-hitting bass you will probably not like the RE272 at all, but if you value the mids and highs and don't care about the bass much (like me) then it's a very good IEM.
The Sony EX600 is the better technical performer, but it's also very neutral like a slightly cold ocean of sound with nothing to fault but nothing to cheer either, it will represent the sounds faithfully but if you want a more uplifting experience the RE272 will surpass the EX600.
It's also difficult to become intimate with the EX600 due to it's expansive soundspace, which leads to it sounding a bit 'detached' at times like you are admiring the sound at a distance, whereas the RE272 is more intimate.
I think the RE0 is overrated and should never have cost more than $50, the RE272 if the engineer in a warehouse in China showed it to me and I listened to it I'd be ecstatic and shake his hand saying he made a marvel of an IEM, but in reality at the asking price of $250, well, I think that's a reasonable asking price for the Hi-Fi sound it offers, however the market is becoming quite competetive now.
Another thing to note about the RE272 is the 'tangibility' of notes, like if you are listening to vocals, then the voice sounds quite 'touchable' and will quiver in volume quite well.
The best sound I acquired from the RE252 was with the Yulong U100 and an extra 75 ohm impedance adaptor cable.
The best sound I acquired from the RE272 was with anything from classical to trance via my standard Teclast T51 DAP.
The best sound with the RE0 was during my initial impressions when I was excited it sounded a bit similiar to the CK10, and then sometimes listening to soft pop music while commuting, but considering the very basic design and rather basic sound I can't recommend the RE0.
With the RE252 now discontinued, the highlight of the Hifiman series is definitely the RE272, I hope it comes down to a more accessible $199 but it can justify it's $250 price due to it's unique character, if you have the cash to burn.
From what I've heard, if I could start my IEM journey all over again and had a spare $300 and only needed one IEM, I'd go for the Audio Technica CK10, we all have different value-systems in sound though, so which aspects of sound you value higher than others will decide which IEM is for you.
If you value neutrality, soundspace, fine-tuning = EX600
Value for money, lucid mids, airy highs, imaging, seperation and precision = UE700
Sound quality, delicacy, sheen, character = RE272
To keep in mind, the primary complaint of the EX600 is sibilance, the highs (around 6kHz I think) are accentuated.
The UE700 has finicky bass, if you listen at low volumes it will disappear, and if you listen at high volumes it will distort, the mids and highs are excellent though.
The RE272 has low volume bass, and the soundspace and seperation is a bit narrow (these three traits are common of all Hifiman's).
So looking at above, you should be able to tell which one you'll like the most in how I've categorised them.
The RE252 deserves an honorable mention as a very accurate IEM in extension, balance and timbre, with a unique shell seen on no other IEM to fit very well in the ear like a custom IEM, however this didn't fit on everyone so the idea didn't work and it's now discontinued.
The Sony EX700 deserves an honorable mention too as it has the best of the EX600 and the RE252 in one package, and with the best bass response of all the above imho, it's faultline is hardly anyone can learn to live with the harshness in the mids.
Purely on a sound basis, since I love imaging, airiness and exciting mids I think the UE700 is my favorite of these, with the EX700, EX600 and RE272 all sharing a close second place, that's only my personal interest in sound talking, views will vary.
One thing all of these IEM's lack is the presence of note-decay offered by the Etymotic ER-4S and especially the J-phonic K2 SP, however the K2 SP fails in not having any realistic timbre, and the ER-4S fails in stereo imaging and tonality compared to the UE700.
Llastly there is the CK10 which has the best imaging and precision of all of them, it's more involving than the EX600 and falls short only in it's synthetic tone in the mids, with IEM's like the EX700 sounding more real and visceral especially with acoustic music.
Now I've included all the noteworthy IEM's I've owned in this mini-review I think, leaving out audition units.
For value, the $99 I paid for the UE700 is the best one, especially since you can reshell and add drivers to turn it into a custom IEM like the UM Mage later (and save money a second time).
If all of these IEM's cost $200 that's too difficult to decide, however I'm most attached to the EX700, and I think the RE272 was very enjoyable like rose petals and iron.
