How do people feel about BluOS?

Blu OS works well for me.  I stream Qobuz and it works well.  There are frequent software updates, which can be a little bothersome, but overall it works very well.  Stable operation and good search and tagging capability.

Roon has a better UI but BluOS is free. What's more important to you?

Depending on your Bluesound device, or the rest of the gear in the chain, any differences in sound quality may be negligible. 

I just got a couple of Node 2is, one for the living room and one for my listening room.  It's my first foray into streaming other than Amazon HD on the phone for headphone listening and use in the car.

I like it so far, especially for the wife.  I set the presets for her favorite playlists, and set her up so they show up on the Harmony remote touch screen with the playlist names.  And the phone app is easy enough for her to use to play other stuff, as it's similar enough to the Amazon HD phone app that she didn't have much trouble with it.  And she has severe challenges in dealing with new technology.

Could it be better? Sure.  Most of the problem is that Amazon doesn't really have an API that I've found beyond the Alexa skill api, which kind of blocks third parties from developing a full featured Amazon implementation.

Hard to believe no comments about sound quality.

is this important?  

BluOS is an operating system.  It runs on the hardware of the streamer and provides the UI (user interface).  It can be built into many different devices, so commenting on sound quality without the context of the hardware it's running on isn't relevant.