I'll just add here that for landscapes, polarizes do not do well on wide angle lenses in general (it depends on the look that you are going after). Lenses wider than about 28mm are too wide and do not allow the polarizers to uniformly distribute the effect across the face of the lens. This is mainly noticeable in the sky - where the blue color becomes very concentrated in one area.
Another downside is trying to stitch with polarizers. Again, its going to be difficult to get a uniform effect across the panorama.