From the Pathfinder Design Team this Friday:

Klars: A traditional klar “counts as a light wooden shield with shield spikes”, and a metal klar “counts as a light steel shield with shield spikes”. What exactly does this mean? Particularly, a klar is a one-handed weapon that deals damage like a heavy spiked shield, and it deals slashing damage instead of piercing damage, so where do the differences end?

A klar counts as a light shield for the purpose of using it as a shield (for instance, it grants a +1 shield bonus to AC, has a –1 armor check penalty, and has a 5% arcane spell failure chance). For the purpose of using it as a weapon, it is a one-handed weapon that deals 1d6 slashing damage, but it is otherwise similar to using a spiked shield (for instance, the damage doesn’t stack with the bashing ability, you lose the shield bonus to AC when attacking with the klar unless you have Improved Shield Bash, and so on). As a side note, anywhere that lists klars as counting as shields with “armor spikes” is a typo that will be handled in the next errata.

The official PDT account also responded to a question from Melkiador:

Melkiador wrote:
Thanks for those klarifications. They do clear up a lot, but…

Pathfinder Design Team wrote:
…but it is otherwise similar to using a spiked shield (for instance, the damage doesn’t stack with the bashing ability…

In which way do they not stack? Does a klar with bashing deal the same damage as a small shield with bashing? Or since the klar counts as benefitting from one size increase from its shield spikes would it still deal damage as one remaining size larger with the bashing enchant?

It otherwise counts as a light shield, so it would be a light shield two size categories larger (specifically, 1d6 damage for a Medium light shield).

There are a number of other issues that were raised, so if you have half a mind (or more!) to use a klar, you should probably read through the whole thread.