MP4 Nation is reliable. The site's owner (Raz) is also a member of the MyMPxPlayer forums, where we help people with their Chinese players, be they legitimate or knock-offs. He has even given players for free as prizes in various competitions held on the forums.
Most clone owners (if you don't believe it there are Onda clones too) brick their players by trying to upgrade the firmware, which should never be done on a clone since you can't find out who actually made it. At MyMPxPlayer we have a firmware database which many people found of help.
Buying a player from a known manufacturer such as Onda you get regular firmware updates that are always guaranteed to make things better not worse.
For $139, and with the SDHC slot you can't go wrong. As for the running time claims, they say they dropped a 3000mAh battery in there. That will make for 8 hours of video for sure.
There are 4GB touchscreen Onda knock-offs going for as low as $50 on ebay. But on most of them the touchscreen responds poorly, the menus are ugly and they have the nasty habit of locking up sometimes. And believe me, on most brand names you pay for the name rather than what's inside, these relatively unknown Chinese companies actually make great products.