The downside is you lose aperture control. You have to manually stop down the lens. The big gain about the adaptor is you can add the fisheye lens.
Are the Pentax lenses that expensive still? You're looking at over $300 for both items delivered. That's enough to get you a complete Bronica camera and 150mm lens from KEH.
The adaptor route is great if you intend to build a kit of P6 lenses. If all you want is one lens I'm not sure it's worth it. The seller doesn't mention which mount. You'll want to make sure it's the P6.