Roon DSP Engine: Any downsides to its use?

Is anyone using this Roon feature?

Any general rules to apply it with least compromise to sound quality?
Good question, I’m not sure either, but would be interested in knowing…

Any downsides to its use? No.

Is anyone using this Roon feature? Yes.

Any general rules to apply it with least compromise to sound quality? No. Buy it and use it.

Earlier versions of Roon didn't have quite as robust SRC software, but over the past year has become rather good. In my own personal experience, its provided a small but noted improvement.

Some of the other controls of the DSP can be helpful, but often with the assistance of measurement tools. If you have room issues, it likely worth looking into.

In ending, Roon sounds rather good and its well thought out interface lets you enjoy your library far more easily.
The OP has asked a question with the potential to benefit a lot of users. Kudos.

Imo, in terms of implementation there are as many caveats and pitfalls with the Digital Engine in Roon as there are purported benefits. Frankly, I can achieve better sound in some cases with the entire suite turned off. Attention should be paid to the Headroom setting in the Digital Engine. I discuss this in my review of the superb Benchmark Audio DAC3 DX and AHB2 Amps. After thorough exploration of the Headroom setting, I do not use the Headroom feature at all, but it has remained OFF; if used at the default setting it is as liable to hinder sound quality as aid it. There may be legitimate cases where the Headroom feature is invaluable, but in my experience to date it is more of a nuisance than help. so OFF it went. I keep revisiting it in different system configurations, and it has remained off.

The upconversion of signals is a mixed bag. At times it is helpful, while at other times it is not. Having used it with no less than three DACs, care should be taken to consider what DAC is being used. Currently reviewing a NOS DAC, and in this particular case I cannot use the upconversion due to limitations of DAC. I also could not use the upconversion with the Exogal Comet, as it creates its own distinct signal. No crying about that for me; the sound with the Comet and the NOS DAC is superb. My experience is that the Digital Engine is not to be considered a foolproof feature. You can mess up your sound with it as easily as enhance your sound. You simply go with the settings Roon provides and you very possibly will have rather poor performance relative to what could be achieved.

Again, DACs react to the settings in Roon’s Digital Engine quite differently. You can likely achieve much better sound with some adjustment, or simply turning it off. This is in no way a dismissal of the excellent work of John Siau and others who were working on elimination of intersample overs in digital signal processing. I am discussing this from the perspective of the end user who seeks to gain advantages in system building. I do not see the Digital Engine of Roon as poor, just potential for poorly set up systems out of ignorance or laziness. YMMV

Let not the reader think I am grousing unduly about Roon; it’s great, and I have wonderful sound. I have omnidirectional speakers singing far better than they ever have. No complaints about the end result! :)

Finally, I need to put a big notice: DO AT YOUR OWN RISK on this discussion. Adjusting the Headroom setting does influence the listening level, and some fools could blare the system with distortion. Sensibility and discretion is advised. 

Thanks for all the great info and advise. 

The only feature I use is the Parametric EQ, to lower an upper bass band by up to 3dB to compensate for excess energy otherwise output into the room by my loudspeakers (by measurement at my listening chair). I notice no performance degradation.

I play my NAS files without upsampling (all are Redbook) with great enjoyment, and have no intention of trying any other DSP Engine features. 

If the Roon DSP Engine allows me to do without an outboard DSP hardware solution to apply a bit room/loudspeaker correction, that benefit and Roon's ease of tagging noncommercial music files, more than justify a lifetime subscription for me.

With a few more months of Roon DSP listening, no downside to the Parametric EQ. I don’t boost bass; rather, I reduce the low end by a few dB. I increase treble above 8kHz by a few dB to compensate for a bit of high frequency hearing loss. Sound quality is uncompromised. I keep the Headroom Management and other features turned off. My DAC is an Oppo UDP 205. Roon has enabled me to add just the DSP functionality I wanted without hardware additions/changes.
I have been using the parametric EQ portion of Roon's DSP for over a year and couldn't get by without it.  I have a 8 db peak @ 42hz that I couldn't get rid of with room treatments or speaker placement.  I didn't want to add another box and cables to my system either.  I only address the bass below 200hz in my room with the EQ feature and don't touch any of the other frequencies.  I researched many other DSP solutions before finding Roon.  I highly recommend Roon and the DSP EQ control that comes with it.