I have the $15/mo data plan for my two iPhones and it's fine. However, we have wi-fi at home (meaning, we use our wireless Internet account to feed our iPhones), so we don't incur any charges against our monthly data plan. But when we're outside the home we use a minimum amount of data just for minor email checking, weather updates, etc. This doesn't use a lot of data. If you wish to download apps or do heavy web surfing outside your wi-fi area, find a wi-fi hotspot and go for it. I convinced my Best Buy store to hook me up with their private wi-fi and I use it often for heavy app downloads.
But always remember, when you're not using your phone, you MUST disable all your running applications!! If not, they "might" pull 3G data even if you're not using the phone! And finally, to be absolutely safe, turn off your 3G and "cellular data" options in "Settings" when not using your iPhone. If you follow all these comments, you'll never go over your data plan limits!