AC Line detection using MID400
Has anyone tried with MID400 ?


No I haven't, but...


  So I would want to know whether really, practically, the MID400 gives
  a steady HIGH or LOW signal?


... the answer is explained in the Fairchild (now On Semi) Application Note AN-3007 MID400 Power Line Monitor on the IC.  

There are several example circuits included. Your answer is that the MID400 can be (but does not have to be) used in a way that its output stays LOW during the zero-crossing of an AC input signal. Read about "Saturated" vs "Unsaturated" modes.

The application note explains that you need to keep the AC input current above 4mA (called Saturated Mode) so that the charge from the photodiode current will leak away slowly enough inside the device that pin 6 will remain a LOW output, even during the zero-crossing of the AC input voltage. See the below extract from the linked application note:



(Image source: Fairchild (now On Semi) application note AN-3007)