I use Mayday's, Phoebus, and Ironhand's, depending on which one was first out of the gate on a new release.
For me, my imagination has its limits. I'm a visual person, and I grew up playing ASCII games. I remember playing CASTLE on an IBM at age 7 going "Man I wish there were actual walls and snakes and swords instead of ASCII."
I don't have any undying love for ASCII, it was a technical limitation that has long since been bypassed, and DF is developed in ASCII because it's
easy for Toady to do and not get bogged down in asset creation. Since there are so many wonderful fans on the forums with the time and the interest to do that, why NOT use a graphics pack? If it's not your preferred art style, that's cool. But I've never gotten the elitism that comes with only using ASCII. That's like saying "Yes, my pictographs scratched into rock are inherently superior to your complete written language scribbled on paper." I suppose if your goal is to play a game that looks like gibberish to the uninitiated so you can chortle at their ignorance, then yes, ASCII is superior.
In the end though, I use tile packs for immersion reasons, not playability ones. The only downside is waiting for the artists to update their packs....but that usually gives me some time to do non-DF related stuff.
If dwarf fortress ever went primarily mouse controlled, I would stop playing it. I know where things are, I don't want them changed.
I find this coming from a DF player to be ironic. Toady makes seismic changes to game play every few versions. I figured you' be used to "stuff changing." And I doubt DF will ever go to a primary mouse format. At worst it will probably be a fully hybridized version.