When I first ventured into Hi-Tec-Cs, I thought my favorite tip size would be the 0.5mm. I've always like 0.5mm tips for most of my pens and pencils, and it is the biggest size of Hi-Tec-C. So I picked up mostly 0.5s. As it turns out, I do love them, but I also really like the 0.4s! Possibly even more than the 0.5s...
The 0.5mm is a comfortable size for most people. It's not so little that it has that slightly scratchy feel (common with the 0.3mm), nor does it give the impression that the tip might bend with a heavy hand. Though all my 0.3s have been solid, it did cross my mind that if I was too heavy-handed, I could wreck the tip. I never have though! But the 0.5mm is certainly more solid.
The bodies of all the Hi-Tec-Cs are the same - pretty unspectacular but they do the job. You can always buy prettier bodies for them and just use refills if you'd like. I kind of enjoy seeing the rainbow of colors through the clear plastic barrels though.
I can't get as precise and crisp lines as with the smaller tips, but it's still great. They never blob ink, and the ink dries very quickly so it's never smeared. It also doesn't bleed through pages - even thin paper like that of Filofax (curse you Filofax!). It's not archival quality ink though, which is a bummer. That's the only thing these pens are missing.

I have been lucky to have no issues with hard starting or dried out Hi-Tec-Cs. Especially with the 0.5s though, they always flow well.
They are getting a little pricy, but they're worth a shot. I think they last a pretty long time, despite being gel ink, which usually runs out fairly quickly. It must be the wee little tips!
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Papericide
Price
JetPens - $3.30
Overall
Yes, yes, yes. If you haven't tried Hi-Tec-Cs yet, you owe it to yourself to give them a shot. Get one of each tip size even. Or one of each tip size AND color. Boy, that would be fun!