It seems stable on both my Windows 7 systems, but I don't think the B+W renders are correct. When a tiff is saved then opened up in PS, LR or even the tiff in SPP the B+W rendition looks horrible and plasticky, whereas the X3F looked rather decent in SPP. This only seems to happen with the B+W renders, the colour tiffs seem OK, as they were in 5.4.1. Be AWARE, there is a pop-up warning saying converting a file to B+W will make it incompatible with earlier versions of SPP.
It's interesting to see just where the high ISO noise is in the X3F's, mainly in the red channel, if this is reduced while in B+W mode, the noise reduces considerably.
It looks like SPP 5.5 is adding Luminance Noise reduction to the rendered tif, at the middle setting, even though it is set at the lowest setting in the Adjustment Panel. That's what makes it look plasticky.
EDIT: it appears that this added Luminance NR is not happening now. I think that when SPP was first installed, although the NR sliders in the Monochrome Panel were set all the way to the left, SPP was still adding NR. When I moved the sliders to the centre then back to the left this reset SPP and it no longer adds the NR when making tifs. Great. BTW this "buglet" happened on both my systems.