For anyone who is on the fence about this, I'll offer my initial take, with the caveat this was my 1st session, which sometimes tells you everything, but sometimes not:) -

Setup the espi with the 11 rack in the FX loop as described here, set the 11 rack global to rig out no cab....

The 1st thing I notice is more clarity in the patches. The sounds are more transparent for sure, more realistic and closer to the real thing. How much closer? I guess that's subjective...but closer. Overall I still find the 11 rack a bit harsh sounding but external IR's help smooth things out. I'm using the stock epsi IR's. I use mostly cleaner and slight breakup sounds and I'll admit that I had trouble finding the right IR for my specific patches. Most of them had more midrange gruff that I needed for my fender type patches. I'm sure there are better IR's for my needs, and I'll probably find some soon.

Bottom line - it's an improvement over the 11 rack stock cab sims. For $150 I get a little closer to having a sound that I like. You could also think of the IRs as a form of EQ, so finding the right IR for your patch was a little time consuming for me...Granted, this is my 1st go around with the espi and IR's, so my experience is very limited, and with further tweaking, more understanding of what I'm doing, and the right IR's it could get much better...

At the same time, I could sell my 11 rack and espi and throw in another $400 and get an Axe Fx ultra for $800 locally...yeah or neah?