Canon for DSLR *only* because I happened to pick up a Canon 300d all those years ago when I got back into photography, and bought into a set of lenses.
Too much invested to change, nothing other than that.
On compacts, I switch all the time based on whether I like what I see or not! Currently it's Fuji, but it has been Casio (a heady 320x160 QV10a), Sony, Pentax, Canon, each many times over!