I've just finished the J5 kit I bought a couple of weeks ago. Somewhat surprisingly, given my lack of knowledge or experience,it even works! No smoke or funny smells, all the voltages look right etc. Most gratifying! And it sounds great.
I found the nost difficult part of the kit was getting the heat shrink tubing onto some of the lugs - and remembering to put it on before soldering! There should perhaps be an inch or two more of the large diameter tubing. And it took me a while to work out what 'drilled wire' is supposed to be. We call it 'twisted' wire in english, but in general the instructions are good and clear.
The is one problem, however, which I was afraid of before even starting the kit: It's too loud! :-\
For home use a 5W amp is still way too loud when cranked, so I'd like to suggest a new kit project based on something like the 1W Moonlight or Firefly projects on AX84.
If you want to reduce the output power of the J5Mk1, you can add a switch at the back to switch from pentode to triode mode. You just need to take the upper end of R21 ("upper" on the schematic) away and connect it to the plate (pin 7) of the EL84 or the original connection via the switch, that's it.