These are, mechanically, among the best retractible gel pens I've used. So I use them a lot, but the large diameter one (which isn't "near-disposable") often have refills from other makers in them.
This is a type of pen I at first I didn't think I was
interested in as I didn't see the advantage with roller
ball gel ink pens.
But after trying out the first one, I quickly came to like
them.
The G-2ex is the one which is a bit wider and with some hardware in metal.
They write not like traditional ball points, as the ink flows much better, nor like a fountain pen as the line width and flow rate is constant, but it's sort of smooth like a fountain pen. You can also use them where fountain pens would be impractical.
So, to me it's not clear if I should consider them a better sort of ball point pen (I really like the Pilot G-2 for being capless and not having a twist mechanism) or a substitude for fountain pens. Perhaps it's actually both, depending on circumstances.
I've read that gel inks (which isn't at all like traditional ball point inks which uses dyes and solvents like benzyl and phenoxyethanol) which consists of pigments suspended in water, write about 1/8 as long compared to ball points. To me, this doesn't matter.
Mechanically I think these pens are a bit better than most others I've tried in about the same style and price range. The refills are good, but not the best which will fit in these pens of those I've tried, but apart from the black I don't think they're saturated enough for regular writing.
G-2 G-2ex
Mass 10 g 15 g
Tip-centre of mass 75 mm 65 mm
Diameter ~60 mm from tip 10.0 mm 13.0 mm
Pushbutton force ~5 N ~5 N
Total length 144 mm 140 mm
Protusion over clip 16 mm 18 mm
You can compare them to the Dr. Grip Gel pen.
Pilot's web page about gel ink pens.