As other reviewers have said, this is just excellent for the money - it would be good value at two or three times the price. It is a very nice looking instrument, the fit and finish is pretty good (albeit a bit rough in places), it plays very well and it sounds terrific.
I have the fretted version (the JB-75MN) and bought the fretless to see how close it would sound to an upright bass, since I play mostly jazz. The answer is not very. I have ordered some Thomastik flat wound strings (which cost half as much as the guitar) and will report back if that makes a significant improvement. I'm hopeful because the Thomastik strings I have on my archtop guitar really brought out the classic jazz sound.
I'm not a bass player so I was expecting the fretless to be hard erto play than the JB-75, but I immediately found it to be easier. The neck seems to be slimmer- it isn't actually, I think it's just that there are no frets to clear and so the string height is lower, and it is very low. I found no problem with getting a clean sound except on the first string, which buzzes a bit. I raised the saddle, which helped a bit. I noticed that there is almost no relief in the neck to I tried adjusting the truss rod, but that made no difference at all, so I set it back to the way it came from the factory.
The fretless sounds better! It has a fuller, warmer sound than the JB-75. The improvement in sound could just be due to the fact that there are no frets or it could be due to one or more of the design differences. The pickups are different - Roswell vs Wilkinson, the fretless has a black wood fretboard vs maple, the bridge is different, with steel vs brass saddles and the fretless neck has a skunk stripe.
There is one issue that I would like to raise which is that the strings do not align properly with the pickups and the neck. The bass string is nearly 1cm from the edge of the fretboard at the end of the neck. I thought at first this was a manufacturing problem but after taking some measurements, I am pretty sure it is because the bridge is too narrow. It has 19mm separation between the strings, whereas the JB-75 has 20mm, and everything aligns perfectly on the JB-75. The photographs on the Thomann website hide this because they are taken at an angle. It's not a big deal, but I don't understand why they made that design change.
Update, May 2021.
I forgot to feedback my experiences changing the strings for Thomastik Infeld flat wounds. Just do it!! I know they cost half as much as the guitar but the difference is unbelievable. The JB-40FL now sounds much more like an upright bass and with the ability on a fretless guitar to slide into the notes, It has quite convincing upright bass character that is ideal for jazz.