The STXI Enterprise is the "least ugly" Constitution for me. It's an alright design, but when your initial material is the TOS Ent, there's only so much you can do to try make it look good without radically changing it (which, personally, I would've been fine with...though I know it wasn't really possible).
What gets me is this bizarre dichotomy on the insides. The bridge and hallways have this very sharp, white,black, blue "future clean" look to them, which I've gotten used to. But the engineering looks like maybe 2 steps up from a 1940s submarine, just....really goddamn huge. Those two looks do NOT mesh. At all.
I love the nacelles (I think they're the best part of the redesign), but I think they should be farther apart in width, farther back (with the pylons more like the Ent-E's) and think the neck should be farther up the engineering hull. Additionally, I think the deflector should've had the TMP-TUC look instead of the silly-looking dish.
At the end of the day, I can take it or leave it. I don't hate it, it's definitely a massive step up design-wise from anything in TOS. But it still doesn't touch the better 24th-century designs. I mostly agree with Saito S when it comes to this ship.