I bought the Boreas not quite knowing what I was going to use it for, and it's become a staple (I now own four!) I use it next to the skin for cycling from spring to autumn, and as a thin mid/outer layer for walking year-round. In both cases it has just enough wind resistance to often be comfortable without any other windproof or shell over it.
I would agree with Mole though that it's not the warmest choice for a mid-layer.
Oh, sizing-wise I'm often a Large (~40" chest) but I happily wear Medium for the Boreas.
The other Rab items that I find equally versatile but a bit warmer are the VapourRise Lite offerings. But the hooded version is a jacket (VR Lite Alpine) and the pull-on doesn't offer a hood.