Recent RE252 owner. I figure I'd add a few comments to the thread.
RE252:
Good:
-some of the best treble extension available. The only earphone I've used that offered a similar sound is the Triple.Fi 10 which I consider a king in the treble department. The RE252 offers the same clarity, edge, extension but is more natural and less "sweet" then the Triple.Fi 10. The CK10 might be about the only other IEM I've used with stellar top end detail, but it's a slightly different sound with better articulation, natural sound, but less edge and sense of limitlessness.
-one of the best balanced earphones I've used with no frequency overshadowing another. Everything blends great
-highly detailed and great dynamic range with the ability to recreate subtlety and nuances when appropriate.
-the presentation is very direct and open. The transparency is excellent. You get a very good sense of directly hearing the music rather then a device playing the music.
-sound stage is somewhat small but there is a great sense of depth and separation. Locational cues are decent.
Bad:
-lacks articulation of note. Some texture and variations to sounds are lost. Many earphones lack this ability though, either through a lack of detail or sometimes being too crisp or short on note to convey a thorough amount of information about the sound. Unfortunately, a sense of reality gets lost from this.
-this earphone is very sensitive to source quality. Poor quality music can make this earphone sound very bad. This earphone really shines with great music. The high level of detail, treble extension, dynamic range, and stage presence scale directly along with the quality of the recording and playing device.
-Tip choice will play a major role. Having a good seal will play a role in bass presence. Having an appropriately shaped and sized tip will allow for very good comfort. An incorrect tip will make this earphone very uncomfortable.
-construction is so so. The housing is plastic but is largely covered by the gummy structure. It doesn't come across super cheap to me though, although there are short cuts with construction including the rounded nozzle edges, the very squared and subsequently hard to use tips, and relatively generic box/enclosure. I've owned the UM3X, SE530, IE8 but also cheaper options like the NE-7M. Very few earphones convey "wealth," and they only two I've ever owned that has was the IE8 and SE530, but that was their design. To me, the RE252 is rather middle of the road, some corners cut (cost choices) but not bad really. I would like to see more reliefs for durability, but oh well. I would like to see better quality tips, and I use none of them. I used Olive tips over the Triple.Fi 10 stock foam tube. Comply T-500 tips would certainly work well, and the RE252 offers enough treble to not be very short using the open cell Comply foam. The hard part is the nozzle doesn't angle up, so the foam can easily bunch up and block the sound.
-comfort and bass performance relies on the correct tip and proper seal. Fitment is gummy + tip, so tip choice is less arbitrary then most earphones. A small or medium T-500 Comply tip can do wonders.
In all, the RE252 is a stellar product that's well priced new and is a steal of a price point (used, street price). I'm certainly inclined to consider this one of the best dynamic driver earphones out there, if not the best. Tip choice will play a huge role in fitment, comfort, and bass. If you want a no hassle option, buy the Comply T-500 3 pack that comes with large/medium/small. Just make sure you fit them in a way that keeps the foam back instead of getting squished in front of the nozzle. A smaller size does this easier. Modified Shure Olive foams or other foams certainly are good options. They just need to be short enough or pulled back during insertion in a way that keeps the nozzle clear. A closed cell type like the Olive will maintain strong highers. An open cell type like the Comply will provide tamer highs and have the mid and bass come across a little stronger. A good seal will yield a significant amount of bass quantity and extension. a poor seal will sound a bit anemic and lacking. Poor tip choice (size, shape, etc.) will also yield less comfortable fitment too.