The J-Head Mk III-B has been reviewed here: http://captain-slow.dk/2011/10/30/j-head-nozzle/
While this review is very positive, there is clearly room for improvement.
Over the past weekend, others have also indicated that PLA takes an exceptional amount of force to extrude. After some experimentation, it appeared that the melt zone should be increased. Therefore, I have machined a small batch of test nozzles that have a small 3.5mm diameter "heat-chamber" machined into them. These nozzles are shipping out for testing today. If this change proves to be an improvement, it will be implemented shortly after the results are known.