Roland Mabo notes at DPR that for the comment from another DPR poster who visited the Photokina Pentax booth that his Bigmas behaved weirdly on a K-5 for the AF. So, is the Sigma HSM just incompatible with the new Pentax SAFOX IX AF system? Does the SAFOX IX uses some completely new algorithms so that the old Sigma in-lens firmware (it's just software afterall) just won't work anymore?
It's not surprised that the compatibility from 3rd party manufacturer is not ensured as they are just not the original manufacturer. Nowadays, even the compatibility between the older and new products from the same original manufacturer cannot be guaranteed sometimes, what do we expect from the 3rd parties? Nevertheless, Sigma in the past did provide some free firmware updates to their products, e.g., their lenses and flashes, to make them work with new bodies from different manufacturers. Let's see if they will do it again this time.
It will not be surprised for Pentax has updated his software, as I believe that the hardware of the SAFOX IX and SAFOX VIII is more or less the same but yet Pentax claims that there is performance improvement. Furthermore, new firmware may work better for the new "DC" AF motor for the DA 18-135 lens, possibly, as this lens is just working in a completely different principle, primitively.
Update: Well, the original tester actually responded in the thread that he found AF problem with his Bigmas on K-7, just, not K-5! :-o So, it seems that it is just an old compatibility problem/performance issue that has not yet been resolved!