At this point in time, one beta tester has tested the J-head Mk V. For a total of at least 6 hours, PLA was printed successfully. He is going to try to print ABS in the near future.
There were two minor issues encountered early on, however. The first issue was that the hot-end jammed. This was theorized to be due to dirt and the hot-end was cleaned out. Afterwards, it printed fine. The second issue was that some PLA was leaking out of the hot-end. After the set-screw was tightened down, the leaking stopped.
As both issues were relatively minor, and were easily fixed, the initial results are fairly positive.
The Mini J-head has also been successfully tested printing PLA and no issues were reported.