I try varnish in one of my server and 
I was really impressed, excellent performances on wordpress and on some laravel applications.

So, even without indications from IPS, I decided to try it:

I've installed a test forum on BBCODE.it
if anyone is interested in testing IP.BOARD + Varnish 
let me know.

Many years ago there was a thread on varnish and the issues they were having. 

Do you have any benchmarks before / after? I think most of the people in this thread are always interested in squeezing out a bit more performance. 

Since most people start moving to Nginx, i don't see any reason to have Varnish in front of Nginx.

In the times people were using the slow Apache, yes, it was worth it.

I'm testing Apache->Nginx/php-fpm->varnish and soon I try to add also redis.