I've bought several JDM motors over the years. All were true JDM motors, in that they were never imported to the US in the original vehicle, but were used only in Japan (and/or Australia and New Zealand for some reason). They all worked fine, but I was able to put eyes on them prior to purchasing to see that they weren't (too) sludged up or varnished or rusty like they'd been sitting at the port for 6 months. I wouldn't buy one sight unseen.
The downside with the JDM motor is that it can be difficult to impossible to source parts for them, which means that it can also be pricey. Mine were all Toyota motors, so there was some crossover on some of the parts with those also sold on US domestic market vehicles -- like the alternator for one of them was also used on the 91-95 MR2 and some of the Celicas and Camrys of the same era. But in my experience, that meant that I had to do sometimes considerable research to see what would work. Accessory parts were one thing, but internals entirely different. Gasket kits can be hard to come by and without an easy substitute.
Having done the JDM motor swap a couple times now, I've concluded that the reputed super-performance bonus that you often hear about is less impressive than advertised, often touted by those who don't have much, if any, real first-hand experience with the JDM motor itself, but are internet experts. YMMV.
Pat Mulry, TARP Racing #67
Mandatory disclaimer: all opinions expressed are mine alone & not those of 24HOL, its mgmt, sponsors, etc.