Pros: transparency, portability
Cons: in-line remote is a delicate flower, MSRP, isolation, clinical
Let's get this out of the way first. The MSRP is a joke. $1299? Ha! Thankfully, on the secondhand market, prices have plummeted below $500, and I got mine on ebay (less warranty, less case, less the two other "sound filters"—I'm not sure which I got, but I think it's 'reference') for $312 including $5 shipping. AKG seems to have admitted this, putting them on permanent sale for $999, and amazon has them for a little over $700.
At $300 secondhand, they're about what my QC25 cost, and although they lack the noise attenuation that ANC brings, I am amid my own internal crisis about whether I like ANC and whether to drop the cash for the PXC 550 or, more significantly, the N90Q. Suffice it to say that although they are not so isolating as Bose (or many other IEMs), they suit coffee shops in a way that HD800—or indeed LCD-XC, TH900, K872, Flow C, or MDR-R10—never will. They are reasonably comfortable, though as with all IEMs that seal the ear canal, you can't use them when chewing. I also find that when I take calls with them, my voice sounds reverberant in the way that it does when I have my fingers in my ears.
The cable is fragile and irreplaceable. Ditto the in-line remote. One wrong tug and you get intermittency. Judicious use of 2.5mm headphone jacks might have yielded a more durable product. As it is, I think AKG is targeting the wealthy among us who can afford $1299 without thinking too much of it. I wish I had the other silicone tips, as the ones that I have are a liiiiitle big for my ear canals at a very long clip, though again for $312 I'm not complaining seriously about missing accessories.
Perhaps the highest compliment one can pay an IEM—indeed, a reference monitor of any kind—is comparison to HD800. K3003i is certainly worthy of being mentioned in the same breath as HD800. As regards SQ, they are inferior in almost every way—though I find bass is more impactful—but not by margins that anyone would find embarrassing. The soundstage (shocker!) is smaller, but it is not mediocre. The tonality is neutral if a bit clinical, and perfect for classical music, my preferred genre, and perfectly fine for the handful of pop music I will tolerate—bad romance, call me maybe, i really like you, let me love you, roar, rise, etc. Treble is not rolled off and the bass is deep and crisp.
HD600, the closest headphone I own in price to what I paid for K3003i, is softer (a critic would say blurrier), and requires an amp (and 4-pin XLR) to sound their best. I got my K3003i for maybe $70 more, and while Bimby/Mjolnir 2 softens the sound slightly, the change is minute. K3003 may change as you change their filters (I didn't receive filters from the janky ebay sale), but in the current filter (which I assume is 'reference'), to my ears, they get maybe 70% of the way to HD800, with most of the deficit being soundstage. Circumaural headphones—and HD800 more than nearly all—can fool the ear more easily than IEMs. Miniaturization is ridiculously punishing, and to pack the majority of HD800's performance into their petite industrial design, and not to require an amplifier makes the technical achievement all the more stunning.
The sound can be unforgiving, and perhaps the treble could have used further tuning. Gidon Kramer playing the Kreutzer Sonata has its harsh moments, though had I been ten inches from the violin, I think the experience would have been similarly uncomfortable. Lynn Harrell on the third cello sonata is pleasanter, not least for my own preference for that instrument. While I can't say that the piano had the slam of the Fostex TH900 or LCD-4, it had solidity and weight—these are reference cans after all. The mids are nicely flat, and there is good detail retrieval; though it is clearly outclassed by HD800, it delivers exceptional performance and is not embarrassed by the comparison.
My ultimate stress test for any pair of headphones is the Solti Ring. Imaging is above average, and at the very top of the pack for an IEM, though (as it's another dimension of soundstage) not competitive with HD800 or K1000. Instruments are indeed a notch less transparent than my top-flight open back flagships, a pinch more distant and veiled. But I repeat: IEMs perform with two hands tied behind their back, and K3003 perform admirably given their physical limitations; IE800, from the makers of HD800, are no better, and (IMHO) worse. Despite all of this though, orchestral colors were handled with aplomb. Brass had its appropriate sheen, depth, and impact; strings soared during O hehrstes wunder and were palpably torn across by their bows during the stormy Walküre prelude to act 1 and Siegfried prelude to act 3. Timpani rumble deliciously but not excessively. Tenor James King was more transparent than Birgit Nilsson; she seemed a bit distant. Wolfgang Windgassen was less immediate (forward?) than on K1000 but K1000 is a magical beast—the forging scene seemed slightly to overwhelm the tenor as it does less on other cans (caveat about herculean demands of the role). But my taste for tilted treble may make my assessment unreliable. Really like Erda. She and Hans Hotter do an excellent duet in Siegfried. Oboes have their utterly lifelike nasality. The triangle is sparkly and wonderful. Immolation Scene comes together very well. Woodwinds, brass, strings, soprano—all coherent and weighty.
IEMs do not have physics on their side. 56mm transducers can do things that IEMs cannot hope to. Despite this, the double balanced armature plus dynamic driver design of K3003i has to be considered a success. Performance approaches that of top flight open back headphones. While it decisively does not equal them, it is a very, very good substitute when you're on the go. Though isolation is mediocre-to-average, and while I might in the future prefer the PXC 550 or N90Q, these are infinitely more stylish and portable.
If you can find them for ~$300, they are very much worth the expense. If they're any more than $500, feel confident in your decision to skip them.