I enjoy mast very much. I'm at the point where I'm not looking to add size, though I get great benefits from it when cutting. If you search online, J3 University has several clients / competitors who, reportedly, made great off season gains by doing nothing gear-wise other than upping their masteron dosage. I haven't tried this myself. The argument people make is that the J3U guys running 1200 test with 1200 mast are...running 1200 test, and that's where the gains are coming from. I think it's more nuanced than that. For a very low aromatizer, EQ and Primo are off the table as secondary anabolics. Lots of guys avoid nandrolone and dont want to use orals in the off season. That doesn't leave much other than higher test with an AI. Also, if you are pushing dosages up high you need to be very mindful of what agrees with your body the most. If say 400 primo works well for you but increased your BP and HCT, and tanks your lipids but 700 of mast does not (it would likely also tank lipids but less likely to impact BP and HCT), you would prob want to use the mast alongside your test.
Is Mast the best secondary anabolic for muscle growth? No. Can it become one for you if you have restrictions on what you can use due to low levels of aromatization and / or health issues from other compounds? Yes. I run Mast almost year round at varying dosages. Love the look that it gives alongside test, and obviously when you add stuff like tren / anavar / winstrol in, you get gnarly looking.