After several weeks of use, I am very satisfied with the JK-9803 sewing machine as it is versatile and user-friendly. I also own a Brother SE600 machine, but after having the JK-9803, I no longer use the Brother machine except for occasional needs such as hemstitching, buttonhole, and embroidery. The JK-9803 can handle a wide variety of materials including thick layers of fabric, leather, faux leather, etc. The knee lifter is a super useful feature that helps free up both hands. The machine is low noise and stable largely due to its iron casting construction and 70 lb+ weight.
For leather sewing, I was able to use a DBx1 110/18 needle and T90 (#92) bonded nylon thread to sew up to 4 layers of 1.3mm thick leather without much issue. For all other jobs, I simply reduced the needle size down to 100/16 and use whatever threads suited for the job. Note the 5mm max stitch length may not be enough for thicker leather sewing due to the presser foot slippage issue as this is not a walking foot machine.
The instructions that came with the machine were not very helpful. I had to spend a good amount of time searching online to gather the required information piece by piece. One of the useful reference documents I found online is the Juki DDL-8700H Industrial Sewing Machine instruction manual (pdf), which contains some critical assembly and threading information.
Note when I received the initial shipment package (2 boxes), the machine base corner was cracked due to the shipping and handling issues. However, after calling Vevor customer service, they sent me a replacement machine without any trouble. I now keep the first machine as a keep spare for parts.
Rich 
XMen 
Frenchy 
