I have a TWSBI and several Lamys, which are great, but the pen I buy and use in bulk is the Uniball UM-153S rollerball. All the fun of a fountain pen without the hassle of dealing with ink. It's the pen for notes and shopping lists. From my Amazon orders, I seem to buy a 12 box of them at least once a year.
To Mitsubishi Pencil Co (makers of the Uniball): please make a green ink version!
Yes I have this set too :-) However it's slightly different from the UM-153S, in particular the nibs are narrower and not quite so nice to write with. (I will admit I'm now being very picky indeed - all of these pens are far superior to biros).
To Mitsubishi Pencil Co (makers of the Uniball): please make a green ink version!