Comply foams are good in three cases:
1) the stock eartips on your IEMs strike you as uncomfortable;
2) you need better isolation than whatever ear tips you are using;
3) you have driver flex issues in your dynamic driver IEM.
Numbers 1 & 2 are pretty straightforward. What is driver flex? Read on.
Driver flex is the dark secret of most dynamic driver IEMS. When you shove the driver deep in your ear canal (or even not so deep) the air pressure can sometimes make the tiny thin little driver inside the housing move a little bit. You can hear this happening; when you push the driver in, if you hear a cellophane like wrinkling noise, that is driver flex.
What happens after the driver flexes? Sometimes nothing, all is well. (However, the long term health of the driver may be at risk. There is no real evidence one way or the other, but anecdotally, many online users claim their earphones had shorter lifespans due to driver flex.) Other times, you will have volume level drop in one or both ears, sometimes even no volume at all in the affected drivers. Other times, you will have bass drop out in one or both ears. These two problems - volume level disparity and no bass - are vastly common complaints in earphone land. How many of these complaints are actual defects and how many are due to relatively easily fixable driver flex? Who knows? Note though that balanced armature IEMs seem to be immune to this issue as they work differently in terms of mechanics and seem relatively insensitive to pressure flex.
Not all dynamic drivers have the flex issue. It depends to some extent on driver shape, then on eartip shape and material, then on internal ear canal shapes for each user. My dynamic drivers from Monster and Shure were never affected by this issue at all. Other models I own from Thinksound, RHA, Velodyne, and MEElectronics have the problem to a greater or lesser degree. My BA models - from UE, Etymotics, & Brainwavz - have never had this issue at all.
Comply foams fix the flex issue by expanding slowly after you compress and insert them. This gradual expansion does not create the sudden pressure changes that can mess up dynamic IEMs. Plus the Comply foam is soft and isolates well.
What's the downside?
1) Attenuated treble, warmer sound. Many people like this combo. If you like classical or acoustic jazz, it makes a lot of music far less enjoyable. All earphones I tried various Comply models on had this issue - T100, T200, T400s on Monsters, Velodynes, Brainwavz, UE, & Thinksound. More bass emphasis, less highs. I absolutely disagree with Comply's statement that there is no audible effect on music with use of their foams. There may not be much effect, depending on your music. Jay Z may sound fine, but Mahler's Fifth or Sketches of Spain will sound much the worse with Comply foams.
2) Hard to keep clean. Like Shure "olives" the Complies tend to soak up ear wax that silicone tips would never attract. This interacts with problem 3 below, as the foams then need frequent cleaning. Plus if you take them out with ear schumz and can't clean them right away, the wax dries and looks foul.
3) Durability. Complys do not take well to the rigorous life of portable IEMs. If you baby them, clean them properly and store your phones in a case, you'll get about 2 months of use before they start to lose pieces and compress less easily. If you shove em in your pocket or knapsack, maybe a month of use, barring acute traumatic episodes.
4) Cost! Do the math 1 - 2 months per pair, 3 pairs per package, $ X for cost and shipping. It's like having a mini cable bill. The stock silicones that came with your IEMs will last six months minimum and are dirt cheap to replace if and when you need them.
The Comply TX series costs more than the T series and adds an ear wax filter screen. These don't sound worse than the T series (i.e. still adds warmth, attenuates highs) but they do add cost for a sort of gimmick. Many canalphones already have a filter or grill across the sound tube, and my relatively waxy ears never produced any accumulation of wax on the internal filters on the TX400. Unless you have a regular candle factory in your ear, the 15% price increase is not justified.
The Comply series is an imperfect solution to some very common IEM problems. Unfortunately, there are not many other universal fit solutions available for multi-brands. Shure and Etymotic tips are much more durable, cost less, and don't mess up the sound as much. The Sony Hybrids are also a fine tip choice. Unfortunately, all these other brands do not necessarily fit brands other than their manufacturers. A secure fit is vital, otherwise the tip gets wedged in your ear and you are off to a loved one with tweezers in hand, or off to the ER if you have no loved ones available. Unless you are positive your Shure Olive or Hybrid will fit your specific Brand X, Comply is the only game in town if you don't like your original fitting options with a given earphone.
Comply has a new TS series tip out with an oval shape (a la Klipsch Image S4s) which is said to attenuate the highs less. Haven't heard them myself, they don't seem to be available on Amazon, and, of course, they cost more than the base models. Considering the unhappy cost to usefulness ratio of the TX series, I would say caveat emptor...