I like AROS because it's free, the hardware's cheap, and it's a genuine community effort so in using it I don't have to feel like I'm choosing sides or supporting crooks.
Perfectly put. AROS and Amithlon is all i am interested in and both run on the hardware i choose at a price i am happy paying.
As for OS4/MOS - I will not support any Amiga (related) companies ever again. they are about as trustworthy as a theif carring a bag with "SWAG" written on it.