for Lumin X1 owners....how good is the unit as just a pure streamer?

Agree, the X1 should be excellent no matter how one uses it. The built in DAC is really good but of course you can do even better if you spend enough and choose wisely. But I wouldn't hesitate to use the X1 as purely the transport if that's your situation. 

I have 3 different DACs in my system, including the X1.   I already use the USB output from my server to another DAC.    As one of my options, I. at times, willl use USB from the X1 to stream digital content to my Makua preamp DAC (Tambaqui)   I  can either use Roon or LUMIIN app from the X1, and I predominantly use the LUMIN app when doing this or using the X1 DAC. Roon is better interface, but sound quality, to me, is better with the LUMIN app.   The idea came to me years ago when there was an associated review of using the X1 to the Tambaqui DAC, and the reviewer being very impressed with end results.   I agree.   I have lots of listening options with the various DACS, types of outputs, software, etc.   I enjoy them all, though different.  I am very happy with the X1.

To update my earlier post, Lumin just announced the U2X as their new flagship streamer.  This looks like it has the same X1 power supply that was previously used in the U1X, but now supplementing the U2 model.