The 3000i is a great high-volume air purifier that I use in the open plan part of the house, as I have a family of asthmatics that are quite sensitive to allergens. I like the display - and the app - and it appears to be highly responsive to changes in air quality and consistent readings with the other purifiers that I have (a mix of Dyson and Xiaomi). However, the non-PM2.5 displays aren't exactly intuitive (you need to know what L2 means for gas etc) and the unit has had a vibration from the get-go. It's not particularly noisy or distracting, but I am aware of it and worry for the fan's longevity, which is disappointing when the Xiaomi models - with similar vertical fan designs and that cost a third as much - don't have this issue.
I also bought the 800 for one of the bedrooms - it seems to work but as other reviewers have noted, is very basic. Fan speeds are basically very low (whisper quiet) or high (noisy) with nothing in between and the auto mode switches between these depending on air quality. Night mode turns the lights down. it doesn't have app smarts or screens to show air quality - just colour rings which are fairly vague. Probably the least favourite of my four different air purifier models.