I get them mostly for free and hardly buy like most users have already indicated. In my collections of bitcoin t-shirt never I have owned any that speaks just about bitcoin as it always get accompanied by either those promoting their service like exchanges, meetups, conferences, wallet providers etc or those sponsoring a charity event like the one I just successfully completed.

Over here in Africa, one of the effective ways of promoting your brand is through distributing t-shirt with you logo displayed on them and since bitcoin is quite popular, most project use it's reputation to promote their brands, like I have t-shirts promoting exchanges with bitcoin logo on them.

I'm also fun of customizing my own t-shirt so guess that's why I hardly buy them too. About what I like most, I'll go for high quality since it means your design will last longer. Giving more attention to design than quality is a waste of money although if justice can be done to both then it's better that way.