Whenever something comes along that I like and/or is a good deal I buy it as long as I got room for it. Depending on its size and condition, I:

a) clean, tune up and ride it (XL, complete and in reasonable condition)
b) clean, tune up and try to get mrs non-fixie to ride it (XS, complete, in good condition AND a nice color)
c) let it sit for a while and pass it on to some interested friend (all other sizes, wrong colors, complete and in good condition)
c) part it out and fill the parts bin with it (any size, any condition)