I played nethack with ASCII for a couple years before I found out about Dwarf Fortress, so I was more comfortable with the graphics than most. That being said, when I telnet to nethack.alt.org I am playing with my nice terminal font; the default ASCII tileset in Dwarf Fortress is quite a step back from that. So I went onto the wiki and found a nice square tileset I liked and tweaked my colours while I was at it. All ASCII. The tileset I went with had diagonal wall corners, because it makes a lot more sense than right angles.
I wasn't totally happy with the tileset though, so I continually altered it, by just a few pixels each time, until I eventually got to what I have now. So now I have something that I feel delivers the visual information of the game in the best possible way, but in no small part because it's a way that I'm intensely familiar with; I don't doubt others find curses to be the best way for them, due to their familiarity with it.
On graphics: If you can cope with items and terrain features being represented by images of completely unrelated things, if you think that it's worth it to have grass that looks like grass and whatever else your graphical pack does? Well that's just fine. But it's simply not possible to convey the same amount of (unambiguous) visual information with a graphical pack as it is with ASCII, and for that reason I will never use one.
I don't use the default graphics because they're not very easy on the eyes. Dat contrast. I prefer Pheobus'.
Yeah, this is mostly due to the fact that the default colours are pulled straight from the HTML standard and are probably intended to be set against white. Modders have created much better colour schemes that are more in keeping with the theme of the game and won't make you go blind (
if you're interested).