Made it to the top of An Stac at the second attempt. First time around, we forgot to turn right at the end of the ATV track and ended up scrambling to the top of Sgurr na ba Glaise, by which time we were too knackered to back-track to An Stac.
If you just want to do An Stac, there is a tempting ridge on the north side but having gone up that way today, my advice is to keep on the Sunny Side. When you get around the north side, at this time of year it is frozen and steep and requires a bit of climbing on some icy rocks - best avoided !
Stick to the ATV track all the way to the end and then take the much gentler (and sunnier) southern slope. From there you can back-track to do the other pyramids if you have the energy.
On the plus side, I saw a ptarmigan, magnificent in it's winter plumage.