I've recently discovered the Pilot Juice pens as I've been looking for the perfect work pen (my fountain pens stay home).
I really like the 0.38mm. Lays down a very consistent, very fine line with no skipping, though it is noticeably less smooth than the 0.5mm. The Coffee Brown and Blue Black are my favorite colours and are a little more distinguished, though it isn't as obvious with the 0.38mm unless you're writing against black lines/print. The black is very deep, which makes the 0.5mm look rather bold even. There is no bleedthrough on even standard paper, but you can see the ghosting from the flipside. The ink does take a short moment to dry, which has been no problem for right-hand writing, but I suspect that 0.5/0.7mm on certain papers may require slowly writing for left-handers.
The spring-loaded clip is something I hadn't considered at all when buying them but now feels like a necessity as I'm constantly on the go (field tech). Clips to my shirt, pockets, notepad, lanyard, laptop, etc... really handy to not have to force it on but instead pinch it open, place it, and release. Capless is also nice for quick easy of access.
I'm going to be trying the 0.28mm Uniball Signo DX, but I suspect that 0.38mm is the sweet spot.
I know this thread is mostly on FP, but it's also on "handwriting" and for that, the Pilot Juice 0.38 is exceptional.
The colours I have so far (and are mostly in the same palette) are: coffee brown, blue black, turquoise green, dark red (which has a bit of blue in it), black, aqua blue, and violet. The colours do appear darker in the 0.5 than the 0.38, and this seems especially true of the lighter colours).
I also tried the 0.7mm for the coffee brown and it's great for diagrams and signatures, making the colour more bold and saturated, but too coarse for handwriting and print, IMO.
Just thought I'd share my experience for anyone looking for a superb daily writer that comes in extra fine points and unique colours that is more practical than a fountain pen. Happy writing!