The most famous user of this setup was Andre Agassi in the 1990s.
This has been on my to-do list for very long, so I strung one up to experience first hand what's so special about it...(secretly hoping that I could return serves like Agassi!)
Compared to kevlar/syn gut, natural gut added a lot more comfort, power and spin.
No break-in was needed. It was comfortable from the first hit. The most impressionable item was the extra amounts of dampening.
With syn gut crosses, kevlar felt a little harsh with a soft "pingy" vibration at impact even with a dampener. Natural gut crosses almost eliminated this ping. Sweetspot hits felt as comfortable as a full syn gut stringjob. But much more powerful.
What I did not expect from natural gut crosses was more spin. Compared to syn gut, natural gut was dry and "gummy". However, the kevlar glided smoothly, adding easy spin to serves and rallies without sacrificing power nor control. That is a huge plus.
The two major disadvantages were rapid fraying and that I could not use it in wet weather.
First pic below was after 3 sessions of about 2 hours each. Visible fraying commenced.
Pic below was after 7 sessions. Spin seemed to decrease gradually with each session but comfort and power was consistent.
Given the marginal improvement over syn gut crosses, I am unlikely to try this combination again. However, I am glad to have this item completed from my to-do checklist!