Well, I have always used and continue to use ASCII(/ANSI/Codepage-437), and so share several contributors' opinions.
I keep seeing that the 'ASCII' is confusing or eye-hurting, but just can't agree. Phrases like 'in the eye of the beholder' and 'horses for courses' spring to mind, of course. OTOH, most times I see screenshots of alternate-tilesetted games I find interpretation sometimes rather difficult. Animals often look like each other in pictures, at least to my inexperienced eyes, and while I can now perhaps see the difference between (oooh, I don't know..., let's say...) ducklings and puppies, where I might get confused where I never did before is telling the difference between llamas and camels and donkeys and horses... that kind of thing.
And, anyway, an ASCII chest/coffer/whatever looks like a chest/coffer/whatever. Levers look like levers (switched one way or another). Tell me that ╤ does not look like a table? Albeit more like one from a diner than a Mead Hall. A ∏-symbol looks like a Dark Fortress's dread portal or a wardrobe, according to context... (And I don't know if tilesets have either separated these out or made some new image that looks equally like both, without basically being this symbol fancied up...)
A lot of the characters that aren't within the plain alphanumeric set (and some that are!) are really quite accurate representations of what they're supposed to be... I sometimes wondered if Toady had looked at something like the "û" character and thought "I know what I'll now implement..!". Alphanumerics are the worst, but letters generally stand for creatures of that letter and water-heights (not on by default) are something you pretty much want straight[1]
So goblins and geese look similar... But are you in the middle of a siege/battle or have received a marauder alert? No, then its probably a goose. And you can always double-check, as you can even if it is in the middle of "the Battle of Goose Green". Pause and check it out with the cursor... You have that right, and would probably benefit from a bit of stop-motion micromanagement anyway, in that circumstances.
[1] Although I saw, in an included tileset screenshot, in this thread a "levels full" version of the numbers. As long as that doesn't make menu quantities/distances unreadable, I like that idea.