0.5mm seems to be almost everyone's favorite. I've been experimenting with 0.3mm, but I will never abandon 0.5mm entirely.
The Rotring 600 and 800 may be the best-looking mechanical pencils on the market. I have a black 600 in 0.3mm, but I keep wishing they would make an 800 in that size too.
I use a rotring 800 at work a lot, it's my standard pencil. I'm a specialized data analyst for a factory, so sometimes I'm out on the floor taking notes and having a sturdy retractable mechanical pencil is amazing. I really want a 600 sometime soon, probably in silver. I love mechanical pencils.