I don't think I've ever used a red ink pen in my life, so I'm rather amused that Muji could have produced a pen that caught my attention with its color and quality. This is one of the Muji pens I bought a while back, a gel ink 0.38mm. I like the feel of the 0.38mm gel inks, they flow very nicely across the page with just a touch of resistance to this Rhodia 80 g/m, and they have a fine line that's readable. Not much of a shadow either, so both sides of the page can be used.
The Muji gel inks have a standard ballpoint-type ink cartridge. This particular red ink is vibrant and a true red. The pen has a polycarbonate shell and is slightly frosted so you can't see inside the pen, and refills are available so you'll never run out of red. Very modern design with a rectangular clip. For the price, about $1.60, it is a great gel ink pen. Can't wait to try the other colors.