MAS does make inexpensive fiberglass parts. That means they are not going to fit perfect, they are going to need finish work to look nice, they will never be an OEM, restoration grade product, and their attaching methods are not remotely close to original. However, if you want an inexpensive piece that looks very similar to factory peices at a quarter of the cost and don't mind doing some work to get them finished, they are a fair value.
I've picked up fiberglass from a number of vendors. MAS does tend to be one of the lower quality suppliers out there, but that is, IMO, reflected in their prices as well. The differences between a $45 spoiler and a $200 spoiler better be significant, and in the case of MAS, it is.