Roon Room Correction- Who Has used it? Opinions please!

AFAIK, you need to get a mic and measurement program to see what the response is. Then you can enable EQ in Roon to change it (say to eliminate nulls). At present, Roon is not a complete solution, as it neither measures nor generates the filters used to perform DRC. It will take filters and apply them.

I have used its EQ for other purposes and found it quite good.

You can use Roon's DSP based EQ without any microphones, however if you want to do actual room correction with any sort of automated algorithm then a mic will be required.

Roon equalization will allow you to adjust frequency ranges up or down, but it has no way of telling you which frequencies to adjust.  I don't know of any way to plug a microphone into the Roon software.  

If you use ROON and want a pro to do the DSP for you remotely for your room (after you measure it). You use some free software and a decent mic for about $100 to measure the room via guided instructions.  

Digital Room Calibration Services, Convolvers, Headphone Filtersets (accuratesound.ca)

The end result is called a Convolution Filter. This is a turbo charged DSP implementation for your room, your gear, and your seating position. The Convolution filter runs on your ROON Core. If you use JRiver it runs on that.

Since this filter runs on a computer there are no limitations on what it can do, unlike DSP built into some gear, such as a preamp.

Highly recommended by me.