They only live high up (above 2500/3000 feet) so us hill walkers are they only lucky ones to see them. So yes rare unless you are a hill walker, not rare if you are up high a lot
Easiest to spot at this time of year as they are still a bit white and you can see them against the rocks. When they are completely white and there is good snow cover or in their summer clothes and no snow they are very well camouflaged - can be heard but not seen!