Are these comfortable to use?
Recently I was stupid enough to volunteer to mark exams. In total I "emptied" 4 of these pens. It still baffles me how people can write 30+ pages in 5 hours without their wrist falling off:
The photo that I posted was a 0.9mm mechanical pencil, but Rotring does make 600 & 800 series pens.
The 600 and 800 series pens & pencils are comfortable to me, but I would not recommend them for long writing sessions. They are solid metal drafting style pens & pencils and thus heavy. If you use them like an architect or draftsman would (slow and meticulous) then they are perfectly fine.
If you need to write fast, then you need something lighter or you will fatigue your hand fairly quickly (assuming you don't have monstrous hands). Jet pens
website will let you filter by weight, grip diameter, grip material etc to really help you narrow down your options.
The Schneider Gelion+ uses gel ink, and because of the inks composition you will always burn through it fast. I'd recommend standard ballpoint ink, or a hybrid ink.
I personally like Schmidt easyFLOW 9000 ink, it's a hybrid ink, and comes as a ISO standard G2 refill.
Off the top of my head, i's recommend