Mitre Cut sails
This is not meant to denigrate the guys at Mack Sails in any way. They are competent and clearly have integrity. I also think the Mitre Cutsails can work just fine, BUT-there is a reason they are the only ones pushing this. This is very old technology which evolved as a way to deal with sail fabrics which were only strong in one direction (old cotton fabrics). The advent of modern fabrics with multi-directional threads has eliminated the need for this type of construction. Provided the sailmaker chooses the right fabric, you will get a good crosscut, bi-radial or tri-radial sail. The tri-radial (and of course newer "string sails") are the ultimate in aligning the strongest threads in the fabric with the load paths, and all other designs are a slight step down (but adequate for most cruising applications).
My feeling is that this is a bit of a gimmick. It is certainly not as effective as a tri-radial, but can produce a decent sail if the right fabric is used.
I am not saying not to buy one, but I am saying that if this were really the answer to sailmaking, you would see the major sailmakers (who have much bigger resources for sail design and cloth development) doing it. None of them are, so just keep it in mind. That said, if you are getting good service and pricing from Mack, you should stay with them!
Happy Trails!
S