I was an owner builder for a house that was completed about a year ago. All the doors and windows were bought from GEM Doors, and their supplier was Southern Star.
The doors & windows supplied were at a better price by a mile, compared to any other supplier, and the quality was neither good nor bad. They do what they are made for, and have not caused any issues - and as mentioned, no one does it cheaper!
The five star review however, relates to a more recent experience where I approached GEM Doors to request cover plates for the wide floor tracks on two stacking doors they had supplied. The cover plates are long strips of metal, that fit into the groves of the sliding tracks, and make it safer to step in and out of those doors. GEM referred me to Southern Star, and on contacting them and explaining what I needed - they traced my original order, and had the cover plates ready to collect within a week, at no charge! No dramas, no shirking of responsibility, just plain old good customer service (thanks Nicole)!