I've been using a Bialetti for years now, and I only rinse. But I am rather sloppy about it, usually not emptying the grounds and rinsing until the next time I make coffee, which means grounds often sit there for 24 hours. Yuck. Oh well, I am a slob.

But the coffee has been coming out consistently good. Or as good as I need it to be, at least. I haven't cleaned with soap ever, so I can't offer a side-by-side testimonial. I doubt anyone can, since most people are one or the other, and you'd have to set out to deliberately do the experiment to arrive at the result.

If you don't clean with soap, you'll notice that in the bottom cup, over time, you'll get little white spots. I am not sure what they are (oxide?), but online discussions seem to indicate that these are harmless.

The "seasoning" by the way, doesn't appear to be of the attractive kind of darkness that you get with cast iron, say. The aluminum just looks coffee-dirty.

If you DO use soap to clean, I suggest rinsing very thoroughly several times, each time, to get all the soap out. The interior of the bottom especially, is not smooth, and may trap soap. My suspicion is that even if you don't get the "seasoning" the metallic taste will eventually go away.

And I'd say never use something like steel wool in a misguided attempt to get the original aluminum shine back...that's a recipe for eternal metallic coffee.