With normal use the iphone battery lasts a full day, but you definitely need to charge it everyday. The alternative to having a replacement battery are the external battery packs that extend the charge. If you use it a lot and are away from your charger/computer, consider an external battery. There are several brands available:

http://store.apple.com/us/browse/home/shop_iphone/iphone_accessories/power?mco=OTY2ODYyNg

If you are concerned with the lifetime of the battery, I have not heard anything negative, I've had one iphone for about 18 months and it still holds a good charge.

The biggest downside of the iphone is ATT. You need to determine is the coverage in your area is OK. Where I live, ATT is fine; and it is adequate most places I travel. Occasionally I find a spot with bad coverage (compared to Verizon), but I can live with ATT.