I really like the Black Axe concept wise, I mean I picture this obsidian blade (and in case you haven't gathered, I love Obsidian...) that, when it cuts flesh, releases little flecks of obsidian into the bloodstream, and naturally these little shards are semi-sentient and remind me of little razor-sharp insects in your blood to deliver pain in future rounds. I was disheartened when I saw the price tag, I figure I would only take it if I was to field a second wight king (and since I really do love Wights, I would consider it) but then I lose some magic because of it... drat this army! I would even love to field two Wight Kings and two Necromancers just for fun some time, not a very competitive list and I know I need a vampire, but hey! A man can dream.
Back to the topic, I have always been weary of multi-wound weapons because of how situational they are. Take, for example, the Chaos Hellfire sword, D6 flaming wounds is amazing vs characters, trolls, treemen, giants, hellpit abominations, etc etc, but if you get stuck in with rank n' file troops all day you have a 100pt flaming sword, something we can get for 10pts that has flaming attacks! I think that a 5+ killing blow is the opposite, as there is few situations where killing blow is NOT useful, such as heroes with armor or abilities that negate it... if the axe was, say, 30 points, I would be down... even with the prospect of GW's providing a +2 while mounted I wouldn't be thrilled by the Black Axe.
Just my thoughts,
-- Ravenfeld