I have had two of the ebay Chinese units, one double din, one single din with flip out screen, so from experience not conjecture:
GPS is equally as good as a TomTom and simple postcode entry.
Menus operate perfectly.
Display is perfectly adequate for dvds, daylight viewing etc
Bluetooth is good for phones, better with external mic.
Fully loaded with everything you could want and more.
My double din is still going strong after 3 years, now in my nephews car if anyone doubts reliability and flip out screen one never cause any problems and its on its second year.
Customer service was 2nd to none also, when i first got the double din the sd card for the maps was faulty, had one sent to my door within 3 days.
Audio isnt as good as branded, but run a decent amp and its just great as its the amp producing the sound, not the head unit. In the past i have spent days installing high end audio and came to the conclusion whatever you did or spent (within reason) you were always battling against the worst environment possible to try and replicate quality sound. So, for what these provide i personally couldnt be happier.