For one, the stabilization is great. You won't see that horrible jello like when moving the GH2 around. With dynamic image stabilization on, do slow or moderate speed movements/pans and the HF M400 will look like a steady-cam. You can also either set it to use the Powered IS to toggle on and off or on all the time when you press the button on the LCD screen for even better stabilization, especially for close-ups.
The auto-focus is very fast when set to "Instant AF". You can flatten/ turn down the sharpness, contrast, etc, in the settings/via the function menu - although I don't recall if that will work in Cinema mode but it is available when you switch it to "manual". Also, to lock exposure, you should touch on the subject on the LCD screen when going into exposure settings and then hit the X to confirm - this way, you won't get sudden change in lighting. Manual touch focus is pretty good, too - with the right zoom distance you can get some bokeh after touching the subject.
You can get really good images, but the main shortcoming of these camcorders is no way to judge exposure; i.e. histogram or zebra stripes. If it's for casual shooting, doesn't really matter - but for anything serious it may look less than professional without those exposure aids.The other thing is the touchscreen menu can get frustrating, because they give you such a small area to land on things - so there are many false hits. You can use the wireless remote or included stylus instead of fingers.
You will want to pick up a long-life battery; the BP-819/about 4 hours and the BP-827/about 6 hours, although that last one will make the cam much heavier.