Pros: Isolation, accessories, comfort, okay sound quality
Cons: Bass, too bright sound, fatiguing at loud volumes
Disclaimer: I use the earphone without an AMP, and the point of reference is a HiFiMAN HE-400i, i know they are not nearly similar, but they produce sound (only similarity) and I don't have another earphone (or headphone) in this category to compare it to. One more thing: I am a headphone guy, the only reason for me having an earphone is the need of on the go and away from home music listening sessions due to increasing addiction.
So, in June, as the summer begun, I knew I was going to be away from home a lot, and won't be able to take with me or use in all cases my HiFiMAN HE-400i, I had to buy an earphone. In my opinion, earphones are the most convenient for portable use because of their versatility. You can use them in a lot of ways and situations when headphones would be too big or uncomfortable (for example: when running). I chose this because of it's form factor and style, the amount of included earpieces, and the way it's worn, which makes it sit very safely in your ear (earphones slipping out was a big problem for me). After that, I started to look for used ones, to get the most out of my money. After a few days, I found a guy selling this barely used SE 315 for half price (purchase price was 107$), it was a no-brainer for me, I bought it. Now, that you heard the backstory, let's get to the review.
First of all, the sound signature: bright. For me, very bright, which has it's certain pros and cons (probably more cons than pros) that I'll talk about later on.
The Bass: It's very odd, for me. The fact that the earphone is bright, should make you assume that it doesn't have outstanding bass. Well, this is correct, but the bass is rather just odd, than bad. This is because it delivers pretty nice, it's tight and has a satisfying humph. Until a certain point, because the extension and depth isn't very outstanding, although, it's quite difficult to find truly good, neutral/warm/balanced sounding earphones in this price range that have good bass and mids/treble at the same time. Also, the upper bass loses it's power and gets a bit quieter as well as becoming rather hear-able instead of feel-able, that is, unnatural, because of this, I feel like the mids and bass are a bit separated, not merging properly, and this can be disturbing. Moreover: the bass doesn't give a nice, enjoyable body, or warmth to the music. It's just there, lying around on the side, alone, with his own little "territory" separated from all other sound. This might be the biggest complaint I have according to sound quality.
The Mids: I am in trouble when talking about mids, because in my opinion, this range is the most difficult to describe without having a respectable amount of experience with different headphones, for example: I've never listened to a Grado, or other mid kings yet :( But, as far as I can tell: little bit messy (especially on lower end, not merging with the bass). It's not nearly bad, it's just might not be as good as the other ones, it's neither recessed, nor raised, seems to be on point. Different layers of sound cannot be differentiated as easily though, mostly due to the nature of earphones, and lack of multiple drivers, the slightest details in that 5th layer won't catch your attention that easily or take control of you, this earphone has only one driver each side and that's just incapable of recreating a sound this complex and diverse. It isn't as good as a headphone with the same cost, but then again, it's "only" my earphone, and I am satisfied with it.
The Treble: It's nice, pretty, sparkly, though sounds aren't pronounced enough, for me. Treble should be the fastest, most responsive, and very strong, distinct, while being as accurate as possible. A lot of treble sounds, for example a cymbal should be a very edgy, very fast and aggressive starting and insanely quickly ending sound, to a degree that when it's over, you start to think about if the sound was really there in the first place. Just come and go like a lightning, by the time you'd realize it's there, it isn't anymore. It lacks a bit of that, though this is in fact a quality of pricier sound equipment (like the HiFiMAN HE 400i).
Other sound attributes: the soundstage is better than what i expected, which makes me happy, because of how important it is to me, you don't feel like the source of the sound is literally in you ear, you can feel it moving in space, coming from different directions, changing constantly with different layers of sound, sometimes you even move your eyes involuntarily to look where the sound is coming from. This is something that not every earphone is capable of delivering, and gets a lot better with more expensive ones. Now, one more thing that might make you reconsider buying an earphone over a headphone is that the sound is not very fluid, you don't feel very immersed with the sound (more on this in previous parts), there isn't enough body and detail to the sound to be able to fully merge the layers together (bass and mids for example) which would achieve a constant flow of sound, without it, it doesn't you suck in to take hold of you to only let go after the track ended and you start to wake up from delirium, it's just not good enough, although, again, better headphones in this price range might be capable of it.
The miscellaneous: The eartips are wonderful and you get a lot of them, so that you will surely find the one that fits your ears best and achieves the best possible isolation. The cable is very very good, it's a normal plastic cable but there is just no problem with it whatsoever, it's replaceable as well. Big thumbs up! You'll also get a nice soft-cover case/pouch, to keep it tidy and together in a compact space, when you take it away and don't want a cluster of cable in your pocket. Comfort-wise the earphone is very good, even outstanding... as soon as you get used to it, because you'll have troubles with correct insertion, and placement in the first few weeks, which can cause pain in your ear, to a degree that it could even discourage you from using it in the future (almost happened to me), even though it's just a beginners lack of experience in correct insertion. This isn't the earphone you crank up the volume with and enjoy music even more, because you will hurt yourself that way, better sound equipment just don't get fatiguing with increasing volume, because of their sound which you rather feel instead of hear, and feelings don't really hurt us physically. And one more thing: take your time and try ALL the included eartips, because finding the right one is crucial for having optimal isolation, comfort, and a non-slippy experience.
Overall, I'm satisfied with the Shure SE 315, it does what I wanted it to do, serves me well in all the situations my beloved headphones can't, and in spite of the worse audio-quality, I enjoy listening to it, some tracks can even sound better with the emphasis on the main melody. You probably won't be using this as your daily driver, either because you are a headphone guy (or speaker or earspeaker, or whatever else) or you really are an earphone-guy and you're using a better one. And while there might even be a better choice for this price, I think this is still a pretty strong competitor for someone who wants a really versatile earphone for activities.
Enjoy your music!