Received both the Cube U9GT5 (Rockchip RK3066) and Nextway F9X (Allwinner A31).
The Cube U9GT5 is definitely lacking in GPU power for such a high resolution display. There's fairly noticeable lag switching screens, etc when in HD mode (2048*1536). The tablet feels like it's running Ice Cream Sandwich rather than Jelly Bean. Forget doing heavy 3D gaming on this device. Things were more fluid in Normal mode (1600*1200) which is good enough for most tasks, but not when reading comics in double-page spread which is my intended usage. That said, it handles Perfect Viewer just fine even in HD mode. Unfortunately, although it's supposed to come pre-rooted (SuperUser was already installed when I received it), root doesn't seem to be working so I couldn't get apps that hide the navigation/status bar working. App compatibility is also pretty bad. Haven't tried installing custom ROMs yet.
The Nextway F9X stock launcher constantly crashed on me but after installing Apex Launcher, it fared much better compared to the U9GT5. Unfortunately, it's exhibiting a weird bug in Perfect Viewer. Every time I tap the screen to go to the next page, it brings up the menu which is darned frustrating and renders Perfect Viewer practically unusable. I'm hoping a firmware update will eventually solve the issue. Aside from that complaint, it works okay. The F9X comes rooted and it actually seems to work on this one. The chipset seems well matched to the hardware but I think they still need to iron out some firmware bugs. I guess it's to be expected given how new the Allwinner A31 is.
My advice, don't get a Chinese tablet and wait for the Archos 97 Platinum HD (Allwinner A31, $299) instead. Hopefully, Archos should have better firmware compared to off-brand and at least you'd likely get better warranty support from Archos than if you import a tablet from China.