I too have wondered why they dont seem to have been hit like other basses, there are some fantastic basses Musicman's etc going for prices almost within my reach!
I am saving as I do want a Wal, I have played a MK I, and I was not that taken by it, but........I only had 5 mins with it, was not comfortable and did not even get a chance to tweak the settings etc.
I want a MKII, in fact, I NEED one lol and one day I will have them, I did wonder if the fact that Paul is building them again, would the used market drop at all, but it has not been affected.
I am a total Tone chaser, obssessed with Justins Chancellors Tone, I have been told its the player, not the gear, but there are a couple of guys on the tube, that use Wal's, and nail the sound!
I also think they look utterly georgeous, I will probably get flamed but I dont think a P or J bass looks "special" as there are so many clones, Stingrays and sterlings looks special, and for the most part are recognisable.
I also think the Wal has a pretty good list of pro/known players compared to other "boutique basses" from what I can see - maybe that is keeping the price high, along with the fact, they dont really seem to come up for sale that often!