Brief Backstory:
I ordered the mic'd version on a whim during the brief time MP4nation had them for $28.50 on Amazon. I figured they weren't too common, and it was almost 75% off MSRP, so it was worth a shot. 
 
On to the Review:
Out of the box, the first thing I noticed was that the cable didn't seem to be as advertised. In the promo pictures, it looks like a sturdier plastic-sheathed twisted cable, but the one I got had rubbery black ones almost reminiscent of iBud cables. After some more research, however, I found out that the mic'd version comes with a different cable, that seems less sturdy, and also lacks a cable cinch. Beyond that, though, the build looks pretty good, with functional, flexible strain reliefs at the both ends, and solid-feeling aluminum shells (though there are minor alignment issues).
 
Accessories are 3 sizes of (subpar) silicone tips that I had trouble getting a seal with in the beginning, and a pair of Comply's S400 foams. These are the newer, cheaper ones, that as far as I can tell use an inferior foam and lack the smooth coating of the T-Series. They got kind of itchy after a while, but sealed better than the silicones. After some fiddling around, I found that Sony hybrid tips gave me an excellent seal, though they had to stretch a bit to fit. Also included is a little molded semi-hard clamshell zip case, and a plastic shirt clip.
 
Fit is very comfortable and seal is very good with the aforementioned Sony hybrids, which also allows for fairly deep insertion, due to the smallish shells. However, the better seal also means quite a bit of driver flex, though only when fitting it in your ear. Isolation is as good as you can get when the housing has that massive hole in the back, which makes it ok for casual use, but not quite as suitable for long car or plane rides. Microphonics are pretty annoying cable-down, but they can easily be worn cable-up, and microphonics mostly go away.
 
Now for the sound. As a couple other reviewers have mentioned, the M4s have a "fun" sound signature, but don't really have a conventional v-curve. Though the bass is noticeably boosted, the treble is not, and the resulting sound is not as fatiguing. The bass extends quite low (pretty much to 20 Hz), and is pretty flat all the way, with only a bit of a midbass hump. As a result, the bass sounds very full and rich, which is nice for bass rumble, but can get in the way of some faster music, where tight bass is more desirable. The lower midrange is a tad boosted as well, and combined with the rich bass section, provides for a nice full tone that, as advertised, seems to work particularly well with acoustic music, and other genres where you want a bit of a resonant quality to the tone. There's a dip somewhere in the upper midrange somewhere, which can take the energy out of some distorted guitars, which, again, makes it a bit less suitable for fast or hard rock music. Up in the treble, it's not as emphasized as the bass, but there's a little hump in the lower treble that gives some energy to the music and prevents the M4 from sounding bloated and completely bottom-heavy. However, the treble does drop off fairly early (I can't tell you exactly where), which prevents sibilance and fatigue, but sacrifices some microdetail. Overall, the sound is centered on the bass and lower midrange, with only a few sacrifices elsewhere. If I were to ascribe it a shape, it would look something like a tilde (~), but rotated clockwise about 15 degrees, if that made any sense. 
 
Compared to the other IEMs I own in its price range (UE500 & A151), the M4 holds its own. Compared to the UE500s, the M4s give up a little of that speed and treble energy for deeper and smoother bass. Overall, I find the M4s a bit easier to listen to, but it can depend on your music preferences. Compared to the A151, the M4 gives up some detail in the treble and midrange, as well as isolation, again, in return for that nice, deep bass, which in the A151 actually drops off fairly early. 
 
Overall, the M4s offer an interesting twist on the "fun" sound signature, offering deep bass without sacrificing clarity. Build quality is adequate, though a little behind what you'd expect for $100, as are the accessories. Fit is good with aftermarket hybrid tips, and isolation is decent. For the going price on Amazon of $70 (for both models), the M4s are a good deal as far as sound goes, and the rest of the package isn't far behind. If you can get it at a discount, though, as I did, it's really a steal, offering solid mid-tier sound quality for the price of an entry-level IEM.

I bought these alongside the UE500, with the intent of keeping the one I liked and returning the other one. Auditioning through a stereo, the S4s were a little less lively and smooth than the UEs, and tended towards some fairly bad sibilance, especially on classical tracks with high violin parts that were almost painful to listen to. I liked the UEs better, and returned these.
 
Despite the sound quality issues, I have to say they were very comfortable and hassle-free, and the build seemed better than the UEs (which have had cable issues since).
 
Keep in mind that this was a while back, before I had a lot of audio experience (still don't), and that neither pair was burned in.

These are my first pair of upper-mid tier IEMs, so I can't really compare these to anything else in their price range. I can say however, that they sound very good to my ears, with much better bass extension than my A151s, and a nice midrange and overall more balanced sound than my M50s. However, they don't have the world's best isolation, and there are some minor build issues, such as the plastic strain relief coming unglued from the housing, though it doesn't really move around and it's easily glued back on. In addition, the strain reliefs on the y-split and the plug are a bit stiff, since the cord is pretty springy (which can be a pro or con, based on preference), the strain reliefs aren't super necessary, but it can still be a bit annoying. Overall though, very recommended!

I'm not too experienced of an audiophile, but out of the IEMs that I've tried, the UE 500s are the most balanced that I have heard to date. Compared to the Klipsch Image S4 (to which they seem to be compared most often, due to availability), the Ultimate Ears are more detailed and balanced. Whereas the S4s seem to have more of a v-shaped (Bass/treble-heavy) sound, the UEs have a more balanced (some might say bass-light) sound. However, though the bass is not as heavy on the UEs, it is very nicely textured and is less boomy than that of the Klipsches. In addition, the UEs provide a very good seal and isolation, and can be comfortable worn over-the-ear or straight down. The only caveats that I have are that A) they can be slightly harder to drive than mainstream IEMs, requiring 50% - 70% volume on my iPod nano 2G, and B) the strain relief at the right-angle plug is nonexistant, leading to some concerns regarding durability at that strain point. All in all, though, the UEs are excellent IEMs, and are also relatively easy to acquire, as Best Buy carries them in their brick-and-mortar stores.