i have an m505. the color screen makes a huge difference. also, more and more new applications are becoming color only.
frankly, i'm shocked at the zire. it's an m100 with a rechargable battery. the new lower priced models seem to be projected at a younger audience than pda's are currently popular with; late teens to early twenties. yet, this demographic (i should know, i'm in it) is more concerned with games than most, and a color screen is the only way to truly enjoy most games.
the only justification for getting a zire as far as i can tell is to get the basic organizer functions; calendar, contacts, memo pad. and for that use, the price seems a tad high to me.