The hardware on many of these tablets can be made to do much more than the often mediocre firmware provided by the OEM enables it to. I ported Android Cyanogenmod 10 (Android 4.1.2) to a RK3066-powered tablet which turned the thing from a sometimes sluggish, rather middle of the road performer to something which outperforms my parents' Galaxy Tab 3 (the 10.1", Intel Atom-powered version). I did the same with one of the earlier Chinese tablets (Ainol Novo 8) with similar results. For the price of a 'branded' Android tablet you can usually get two or three of these, if not more. Given the right attitude they can be made to perform like one of the branded tablets, with the added advantage of the usually larger range of interfaces and expansion options on the 'cheap' tablets.
The conclusion is simple: if you want a guaranteed 'out of the box' experience these cheaper tablets are hit and miss. If you are willing to do some work you can do really well with some of them.