At Opeth's headlining Prog Nation show in Pittsburgh, I bought a Watershed tour t-shirt printed by O-Merch. Being very picky about clothing, particularly shirt quality -- e.g., how fine is the material, how easily does it fray, how clear is the print and how does it hold up -- I was pleased to find that this shirt's material is decent, its print quality excellent, and my expected longevity for them pretty great. It's even tagless, as I therefore expect all other O-Merch is; where the tag would be is a gold print of the O-Merch logo and other tag information.
I'll note that, while not uncomfortable -- more so to my happiness -- these O-Merch shirts are slimmer-fitting than most 'other' shirts of a given size; my small Prog Nation tour tee is nearly second-skin across the chest (probably 36") as opposed to the more generous (~38") size small cuts an American Apparel, Fruit of the Loom, or shitty Gildan shirt would provide. Indeed, an O-Merch medium is 38-40", so a small like that I bought would be 36-38" or smaller. So, some might want to order a size up.
I'd say any purchase of this new Opeth merchandise is worth it. O-Merch was born literally out of Mike's displeasure with "shit-quality" bootleg Opeth apparel, so even just in theory you shouldn't have much to worry about.