I have been using the Pilot Envelope Pen in both the Fine and Extra Fine models for many years. These are rolleball pens. I’d most likely compare them in feel to a Pilot Precise V5 but a little bit slippier. The ink is silky smooth but because of the ink, the tip can get a little gunky. So why suffer a gunky tip? Because these pens are fine line AND waterproof and not prone to feather like a Sharpie Marker. In my writing test, the water test portion was wet with a water brush and dried. Not even a smudge!
The ink does have a bit of an odor but no worse than an alcohol-based markers and there may be some show through or bleed through on some papers but for envelopes or file folders, its not a big deal.
These do use ink quickly so buy two at a time. Stock up now before holiday card season.
I like the extra fine better for addressing envelopes since its easier to write long addresses small and neat. The fine is quite a bit broader.
Each model is just $2 and there is also a broad version and available from Jet Pens.