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.
Actually I rather like the minimalist approach:
-smaller is good
-more battery life is good
-cheaper is very good (and makes me less worried about losing it)
-recharging through USB... great!
-speed is adequate
-includes infrared... so I can use it to move data from my Mac to my phone
-memory is enough... I'll be backing up to my Mac anyway so I only need current info on it
-no backlight makes it smaller... and I don't see myself trying to use it in the dark anyway
Plus... it looks really cool! I think I'm sold... but I'll have to actually hold it before making the decision.