Well I think the ASCII has a certain charm, and I got used to reading it fairly quickly, but when the same symbol is used for Goats, Groundhogs and Goblins, Hippos and Horses cannot be easily distinguished without tabbing over them, and Minotaurs, Mudmen and Mules are all a brown "M", I daresay there are issues that need to be addressed.
It's weird. I've never had any problem identifying the creature I'm looking and knowing what it is based on where it is in the world (goats are white and generally stick to the mountains, groundhogs are brown and live in lower biomes, and goblins never show up unless there's been an announcement). I try to keep tabs on which creatures belong to the biome I'm playing in by checking the unit list regularly fairly early in the game, so I'm rarely caught off-guard (hoary marmots and macaques screw me up on occasion).
I guess I've got to be in a pretty small minority in not getting tripped up by that, given that I've seen any number of people voice their issues with confusing creatures with one another, so it's clearly something that needs to be addressed at some point. However, with the introduction of randomized creatures, you're still going to need to tab over creatures to really know what you're looking at, and making unique tiles for everything is going to be difficult.
For the record, I like ascii, but I grew up playing all the old classics (Including the original Rogue) so reading the "Matrix code" hasn't been a problem for me for a very long time.