I recently bought these three products of YoungJun MGC:

YongJun MGC 4x4 Magnetic 4x4x4 Stickerless Speed Cube
YongJun MGC 5x5 Magnetic 5x5x5 Stickerless Speed Cube
YongJun MGC 6x6 Magnetic 6x6x6 Stickerless Speed Cube

The first and the second work quite well compared to the price.

The 6x6 is almost unusable, especially the middle movement in any direction:
The filling is kind of could brake or pop out some pieces.

Greatcube

Great cube, might need a little lube out of the box, but I ve never had any major problems with it. Sometimes the corner cutting can be strange when you cut it too wide, but i have never popped a piece

Amazing cube, but really fast. Cubezz packed the cube really good and it came after 15 days.