This subject comes up so much, that it's time to really go into the matter, as objective and unbiased as is humanly possible.
First issue:
"U87s are honky in the mids" and:
"but, they still sound quite unique and are universally usable"
My explanation is simple: The honk comes from severe negative feedback, in the high and low frequencies and smaller global (gain reduction) feedback. Phase shift is the audible side effect of this, and is universally disliked in almost any mic by almost any user.
It is interesting to note that the amount and implementation of the severe
negative feedback is identical in both versions, though the global feedback was removed in the new version (which, in theory, should make that mic sound better.)
Despite the feedback-induced honk, the saving grace for this model is the extraordinary good capsule design of the K67/87 which mitigates enough of this garbage that the net effect on the listener is still positive.
Second Issue:
"The old U87i model sounds smoother/better, more vintage, than new U87Ai model"
Let's look at the exact (and only) differences in the two models:
A. 60V pol. voltage in the U87A vs. 44V in the U87.
An argument can be made that the higher voltage does a couple of things in the new mic: it hardens the sound (60V pull more on the diaphragm than 44V, making it a bit stiffer, and, from experience, a bit more aggressive sounding)
The higher capsule output also overloads the FET a bit sooner than the old U87 with its lower voltage into the FET's gate/input.
B. Capsule differences.
The method of spacing the diaphragm 40