Which one in particular? Any?
I have bought 3 or 4 over the years, and used many, many more. They, without fail, have low quality LCD screens, glitchy firmware that lags, the music interface is generally absolutely terrible, video is massively hit or miss. Plus, they are never going to get a firmware update, and if there was a new firmware, you have no way of finding the right one for your unit.
Game emulation? They generally throw a few terrible emulator ports on it without optimizing them at all. Most of the ones that claim to emulate all kinds of systems can only emulate NES, and the ones that go higher do an absolutely terrible job. Sound is broken or non existent, motion is choppy, games are broken, etc.
Build quality is usually terrible, with the case flexing so much, that sitting with it in your back pocket will probably break it. The buttons ALWAYS suck, as they use either low quality metal hat contact switches, or really stiff rubber membranes. The dpads, without fail, have no pivot point, making diagonals impossible, or even worse, the dpad is not even a single piece of plastic, but just 4 independent buttons.
I could go on and on, but the bottom line is that usually, the hardware, software, and support is trash for these generic PMPs. They have been around for years, and they are not even close to the Dingoo. The JDX 301 is really nice, though, and the SmartQ T5 II. Those are the only chinese gaming PMPs out there worth getting.