I've had the 400S working for a few days and I am as impressed as I was with the 200S.  I love how quiet it is, the Auto mode, and laser particle detection.  Removing 99.9% of 0.1 micron particles is awesome.  The HEPA technology sounds good - not sure how I would validate it?

I love the display - very modern and slick.  The App works great and I think my issue with Alerts is probably a user error :-(.  This is my third purchase and I love all the models; they are all being used and we are very happy with the results.  Setting schedules is also so easy and convenient - especially when one unit is remote.

I highly recommend Levoit Purifiers based on several months of usage with the 200S.