I much prefer ascii to any graphics tile sets I've seen. With ascii, it is really simple and easy to tell what everything is. It has a very clean, crisp look. The problem is, there are so many monsters in most roguelikes, that with tile sets I'm always left squinting at the screen trying to figure out if that thing is a pink jelly that saps your strength (and thus is best avoided or killed by missiles) or just an ordinary blue jelly that I don't mind beating up in melee.
Plus I haven't found a graphics set yet for any of these games that does not look so amateurish that it annoys me more than just playing the ascii version. There is something very simple and refined that comes with ascii, and I don't find it difficult to play at all for some reason.
I think we recently had a thread that devolved into roguelike discussions, but I am heavily playing right now Oangband, Sangband, and Dungeon Crawl. Truly the best of the best that I've played, after playing some others and researching many more. (Including Nethack, which in my mind is living on legend alone and is far inferior to the above mentioned games.)
I actually occassionally think of how much of an idiot I am. I'm paying $15 per month for an Everquest subscription, have Dawn of War, Rise of Legends, Civilization 4, Battlefield 2, and probably about 5-10 other "modern" games that I spend a total of 4-5 hours a month playing, and then I sit there plowing about 10 times that into a free game.
EDIT: Though I keep forgetting that ascii graphics on a "normal" dungeon only roguelike like Angband or Crawl look a hell of a lot cleaner and better than they do on something like Tome or Dwarf Forest. Good lord, after checking out their screenshots, I understand what you ascii haters are saying if you are talking about those types of games.