in the internet , sometimes u can say its  small , but sometimes also can say is a big world . thats how you use it.  500 MB easily pass using youtube, download apps , push notification, emailing , IM  and or download games using iPhone .
but if you are a mild user , only take the phone to surf when needed , then is okie. with 500MB u can surf alot of time in lowyat.net .

always think , you are using "smart phone" not a camera phone that can only surf WAP site or sending MMS .

but dont get me wrong , Im still a maxis user , but I only think they giving not enough for a smart phone user .