Compare them to the isolated forms at the Ring of Brodgar, in the Orkneys, where we found no such large, complex forms as at Callenish. See http://www.mysteriousbritain.co.uk/scotland/western-isles/featured-sites/callanais-callanish-stone-circle.html
Ring of Brodgar, Standing Stones, Orkney
See //www.scotland-inverness.co.uk/c-nish.htm. See also www.stonesofwonder.com/callanis.htm and //www.isle-of-lewis.com/history/standing-stones.htm".
Either way, we preferred the wild setting of the outlying stones, rather than the manicure of Stonehenge. There also is a comfy visitors' center here.
Viewpoint: These things are more impressive when it takes some pilgrimage to get to, and not just be available for an easy drive up on the motorway. Walk out there, feel the wind and the salt. Think. Fold your arms and look out a long way back.