Pros: Extremely competent bass but very well controlled; Surprisingly good detail and separation; Not fatiguing
Cons: Not analytical, and the bass does on rare occassions step out of line. Mid-range is a tad recessed.
The CKS99s are, without a doubt, a consumer-oriented headphone in its sound signature. However, despite the prominently advertised bass and the definite emphasis toward that end, the very well-controlled lows rarely step out of place.
First off the design and build. Like other members of the CKS lineup, the CKS99s are a half in-ear design. The shells are larger and stick out a bit when wearing them. They are made of metal, but also high quality soft plastics where it comes in contact with your outer ear. The cable is thick and slightly rubbery and generally very nice and terminates to a compact L-plug. Overall I think it is quite well built.
I found the design to be quite comfortable. Like other half-ear designs, the shell nests in your outer ear. They were quite comfortable for me, though people with smaller ears may have some difficulty.
Given the half-in-ear design, the isolation is quite good. It comes with four pairs of silicon tips, and I use the second largest. They isolate almost, but not quite as well as my more traditionally designed earphones, such as the Hifiman RE-400s. That said, I've never been a fan of double flanges, so I am generally satisfied with a decent single-flange seal. Those who prefer deeper seals may have problems getting them due to the design. Cable noise is also not too prominent, given the half-ear design.
The package also comes with a nice leatherette case. Of all my earphones, this is my favorite case because it uses a very convenient magnetic button rather than a zipper.
I've owned these for about six months now, so they are well burned in from use. The CKS99s advertise its solid-bass feature and delivers on that front. However, I would emphasize that the bass is very high quality and the "solid" description is quite accurate. I don't consider the bass to be very boomy or splashy, but rather very well controlled. In fact, there does not appear to be a lot of sub-bass emphasis. Instead, there seems to be a bit of an upper-bass hump, which makes the music sound more substantial (or thicker for those who don't like that sound). I use these for a variety of genres, from indie-pop and indie-rock to mainstream pop and hip-hop, along with some more acoustically inclined songs and instrumentals. They are definitely great for hip-hop and mainstream pop, but even for my other genres, I don't feel like the bass gets in the way.
The rest of the sound is quite good as well. The mids are slightly recessed, but does not sound veiled and is still quite detailed. The presence of the upper-bass helps to prevent it from sounding hollow. The highs are a bit more prominent than the mids, and surprisingly detailed with good separation. Even when I'm listening to music with little or no bass, the level of detail and instrument separation is quite impressive. The details and crispness is retained even though I would not categorize these phones as bright. I am somewhat sensitive to brightness and fatigue, so I appreciate that I can wear these for hours, though admittedly, I recall them being a tad bright before burn-in.
Overall, these are probably my favorite earphones in my possession. They have a definite consumer-oriented v-shaped sound with bass (the low end is more emphasized than the high end), so they're not going to be analytical, but they're definitely fun and have some very good qualities than just good bass. For the price, I think they're a good value, though this price range has many good options.
Quick comparison to other IEMs in my possession.
Hifiman RE-400s: The RE-400s are a completely different type of IEM, though the price range is somewhat similar. The Hifimans are very neutral and analytical compared to many IEMs in the price range. While I praised the CKS99s for their detail and separation, I think the Hifimans are even better in this regard. The Hifimans are also quite fast and nimble, though the CKS99s are not slow by any stretch of the imagination. The bass in the CKS99s are obviously more prominent.
AKG K374: Also in a similar price range. Like the CKS99s, the AKGs are consumer oriented with a bass emphasis. In terms of bass quantity the AKGs and the CKS99s are close, but the CKS99s are much better controlled. The AKGs are less mid-recessed so in that sense it's flatter and they sound less dark, but they are definitely not neutral by any stretch of the imagination. The AKGs also trail the CKS99s a bit in terms of detail and separation, but the AKGs are not bad at all in this regard.
Klipsch S4: The Klipsch are mainstream favorites, but I would recommend either the AKGs or the CKS99s to a mainstream user before the Klipschs. The Klipsch can be appear quite detailed, but it's more of a function of brightness. They are also quite fatiguing. The highs and mids on the S4s are harsh compared to the CKS99s. The bass is also much looser in the S4s than the CKS99s.