Performance
I tested the KA7 against another recent KX133 offering, the K7V from Asus. My full review of that board is here, if you're interested. This is a performance comparison at mostly default speed. I say "mostly" because the CPU is at the default 100MHz, but the RAM is at 133MHz using the "FSB+PCI" option. Tull testbed specs are:

Hardware:
Abit KA7 or Asus K7V
AOpen HX-45 Midtower Case
Leadman LP-6100 400W PSU (review here)
AMD Athlon at 750MHz, L2 cache ratio 2/5 -OR- 1GHz, L2 cache ratio 1/3
128MB KingMax PC133 CAS3 RAM @ 133MHz, CAS3
Diamond Viper V770U TNT2U Video Card

BIOS:
KA7: KA7_42 (beta), RAM timing "Turbo", Interleaving 4X
K7V: v1005, RAM timing "7ns" (fastest), Interleaving AUTO

Software:
Win98 SE
VIA 4-in-1 v4.17
Detonator 5.16

First up, Sandra (download it here) CPU and MultiMedia benchmarks:

As you'd expect, not much difference between the two boards. In fact, I would say there is NO difference at all.. a few Sandra points here and there means nothing. The Sandra memory benchmark below shows a slight difference but, again, it's pretty small. It's odd that the K7V actually got slightly faster at 750MHz than 1GHz. My guess is it's something to do with the lower L2 cache ratio.

Anyway, onto a more real-world benchmark, MadOnion.com's excellent 3DMark2000:

At 1024x768x32, the results are pretty much dead even 'coz we're hitting the wall of fillrate on the TNT2 chipset on the video card. The results tip slightly in the K7V's favour in lower resolutions at 750MHz, but it's clear the boards are pretty evenly matched.

Conclusions
Well, in my experience the KX133 boards all seem very close performance-wise. I think making a purchasing decision based purely on a perceived performance difference between these boards at default clock would be a bad idea. I'm sure that with future BIOS images we will see the performance lead swing from one board to another, and new features and tweaks may become available. However, the ability of the KA7 to go well beyond default clock and be stable, coupled with the massive configurability, makes it a very attractive option. The Abit's not cheap, at AUD$350 fairly expensive in fact, but it has features you simply can't find on any other Athlon motherboard. If you're building an Athlon system for your grandmother (who can't get enough of those knitting simulations) you'd probably be happy with any KX133 board, but for the overclocking community and performance tweakers (which represents a pretty large proportion of people buying Athlons), I have no hesitation recommending the Abit KA7 as THE board to build an Athlon system around. Thanks Abit, the wait was WELL worth it!

Addendum: I have created a dedicated forum for discussion of this motherboard. If you're a current owner or thinking of becoming one, come join the community!