Who is using I2S cables?

The i2s connection used between boxes is a buffered version of the internal i2s signals. As @lalitk mentioned, there is no standard for the pinout, so you need to make sure the two devices you are connecting use the same conventions.

Some DAC manufacturers, such as the Denafrips DAC I am using, have a programmable interface that allows the interface to be configured to work with different conventions. 

The advantage of i2s over other synchronous digital interfaces (SPDIF, AES, toslink) is that the clocks are separated from the data.

I am using i2s between my Denafrips Gaia DDC and Denafrips Terminator Plus DAC. I found this interface to be a very noticeable upgrade from using AES, even when using a very inexpensive HDMI cable. 

There are audio cable vendors that are designing cables specifically for i2s. Whether they are better than a good HDMI video or Ethernet cable, I can't say as I haven't done a lot of comparisons. 

I am using a Tubulus Argentus cable based on recommendations from other users. I found that this cable provided a subtle improvement over the $12 HDMI cable I bought from Amazon, but no where near as big an improvement than switching from AES to i2s in the first place. 

I noticed a significant difference in my system switching from using an AES connection (DH Labs D110) to an I2s connection, initially using a basic $12 HDMI cable from Amazon. Switching from this cable to the Tubulus Argentus cable was an improvement, but more subtle. If I was on a tighter budget, I might have returned it, but I do like the sound. I have not tried other higher end HDMI cables. 

I currently have my Roon Server connected to the Gaia DDC using AQ Diamond USB. Inserting the Gaia instead of connecting the Roon Server directly to the DAC was a worthwhile improvement. I have not tried connecting the Gaia to the DAC with USB also. 

@donruydesilva - I had the same experience as @soix using my Denafrips Gaia and Terminator Plus DAC. Even a low cost HDMI cable was a nice improvement over AES and USB. 

Unfortunately, Denafrips's latest FPGA update for the Terminator Plus makes the i2s no longer compatible with the Gaia. I have a very early Gaia which does not have programmable i2s outputs, and for some reason, Denafrips chose to change the i2s format to be incompatible with the Gaia. I'm a bit peeved about this, but I do like the FPGA firmware update which they provided for free. 

I am currently rethinking my digital chain, so haven't invested in upgrading the Gaia yet. But I would second @soix's recommendation to get a cheap HDMI cable and give it a try.

@marco1 - yes, I agree. I am currently using the Gaia with AES3 which works well, but I think an Iris connected over i2s could still be an improvement. 

I am playing around with a DSP active crossover (dspNexus by Danville Signal) which would replace my T+, so I'm not going to invest in another DDC until I decide which direction I'm going to go. 

@marco1 - It is available, but it is not in full production yet. They are still working on the software and an upgraded DSP module. 

I do have one and have been playing around with it for the past few weeks. I haven't had a lot of time to spend on it yet so I haven't gotten very far. In addition to my work, I have a couple other DIY audio projects that are taking up some of my time. 

I'm planning to attend the Capital Audio Fest in a few weeks. Al Clark (the head guy at Danville and HW designer) and Richard Hollis (who has done a few active crossover implementations with this unit) will be there and I'm looking forward to discussing the dspNexus with them.