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. 

Currently running an I2S between PS Audio cd and dac. To these ears it sounds superior to the xlr or coaxial connections. The I2S cable in service was the one provided with the unit from ps audio.

I have a Jay’s Audio CD transport and DAC and use an AQ cable for the i2s and it works well.

Thank you @jaytor . And yes, I understand a little bit re. the different configurations @lalitk mentioned. Which is why I was asking for smarter folks recommendations :-) 

If I read correctly, you noticed a significant improvement from AES to a $480.00 I2S cable https://www.highendcableshop.com/product-category/i2s-cables/ ?

So, two follow up questions: 

1. any chance you had an "A-B" experience with a high quality USB A to B connection and is the upgrade expense that big an improvement? 

2. re. a properly configured high end HDMI vs. the Tubulus Argentus I2S, you could tell a significant -- i.e., it was worth it -- improvement? 

Thanks much! 

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.