I have three of these shirts, and I want more!
The number one thing about a Buttercloth shirt is the feel. I've never felt any piece of clothing this comfortable. I'm not exaggerating when I say that putting it on for the first time was an experience. It's both stretchy and breathable, so you feel almost like you're not wearing a shirt at all. I usually walk around my house shirtless, but when I put this shirt on after getting home from work and finding it in the mail, I didn't take it off until it was time for bed!
The second thing is the fit. I'm tall and fit/muscular, so I usually have a really hard time finding shirts that fit right AND are long enough. Buttercloth shirts fit my shoulders, torso, and arms well, without compromising on length. I can actually raise my arms without immediately showing my midsection.
The final thing is, "Great, but how does it look?" The first time I put on a Buttercloth shirt, my fiancée could not stop smiling and touching me. Need I say more?
TLDR: Nice fit, elastic feel, breathable, good length, looks great.