Internet has been reported to be slow using AT&T's network - mainly because the phone is not 3G capable. But, reports have also said that it works marvelous on wifi - so, if you have your house setup with a wireless router you are good to go at home, and nowadays there a plenty of places to find wifi access around town (airports, cafes, etc.).
As far as additional costs - there is a $36 one time activation fee. At sale AT&T loves to add-on their features, like roadside assistance, etc. These features are usually free for 30 or 90 days but then you get charged for them if you forget to cancel. Other than that, the only other costs you should incure are those related to accessories, and blinging out your iPhone (God, I wonder how long it takes for us to see one with all those crystals).
Mike