I don't think I've noticed any HX400 users around here.
At the DPreview.com Sony Cybershot forums, there seem to be several happy owners.
It's a competitive, full featured, small sensor mega zoom point and shoot camera.
I believe some of its competitors (Canon, Panasonic, Nikon..) offer a bit better critical IQ (especially with RAW that the Sony is missing) but the overall performance and features of the Sony work better for some owners. With the hot shoe (for flash or mic) and the Bionz X processor with Alpha-style menus it does have enough control options to play with.
The general point of a mega zoom is...the zoom, so it's a popular category for birders...but you get that zoom because it has a really, really small sensor. (IQ quickly drops indoors and low light). The RX10 has much higher IQ because it has a much larger sensor (and $$$) but less zoom.
If you do not plan on using all of the zoom, the RX100 has much better IQ due to its larger sensor (despite being a much smaller camera)
Here is a smaller mirrorless camera with the same IQ as a DSLr for the same money
http://r.ebay.com/wxVGQg
(It is a refurb and I have not investigated that seller)
Sorry, I always go off on a tangent with mega zoom cameras, it seems people like them because they "look pro" but they are in no way pro. It's fine if you like the size and ergonomics over a smaller point and shoot, but it's not really better than a small point and shoot.