The quality is crappy. The battery is usually less than 5h. They can be hacked to show more than it's real memory capicity. And then it just freezes so you need to formate it, and when you do so, it will brick. And then it's very hard to find the correct firmware for the player.(This happens most to hacked "4 & 8Gb" players) The video playback is very crappy, amv format and only 18 fps (or 16 i'v forgot) makest the video choppy and low quality. Some of the MP4 players have a 2.5mm audio jack(ugh I hate that). The screen is not so good, 65k colours. The players are usually slow and unresponsive, and the browser function can be useless sometimes.
I had a Nano Clone MP4 that was 'ok' but the battery was teribble and such. Now I got a cheap MP4 player again that I bought on eBay for $30, 1Gb. It works ok. But I can't stand the ugly GUI, crappy battery, cheap quality and so on. A Meizu is so much better.
There are Rockchip MP4 players(that play divx) that are better than "regular MP4 players"...but still, a Meizu is a lot better.
After I bought a Meizu, I said no to cheap MP4 players.