I always wear a merino T or merino longsleeve base layer under my Special 6. I like to run cool when hiking and will often remove the S6 and hike in just the base layer, even in winter. In fact, for summer hiking when the S6 rarely comes out of the pack, I just have my merino T. As far as waterproofness is concerned if it's light or intermittent rain, or a short hike, I don't worry about a waterproof layer over the top, but heavy and persistent rain or long hike I will generally have a cag or my Jerven bag on top as my wind and waterproof layer - purely to keep some gear dry for when I stop.
I think it's a great bit of kit. Been using assorted Buffalo gear for 30 years or thereabouts...