I toured the Martin factory in 1976 and saw literally piles of Sigma guitars at the factory that were inspected restrung and sold to Martin retailers with CF Martin backing. Any idea that they are anything but imitations of the real thing ie genuine Nazereth made Martins, is fanciful. But as Leadfingers points out they are are very workmanlike instruments.
The tonal deficiencies will be exposed when you take a Sigma into a recording studio and discover the absence of the extended harmonics that are the halmark of genuine handcrafted Martins and the other quality makers like Taylor, Santa Cruz and Collings especially. But as a budget instrument that plays well and does the job they are excellent value if carefully chosen as the quality varies greatly.