I had a 3GS and decided to not use my previous sync/backups, and just saved my music/pictures and apps through itunes and set it up as new. I am thoroughly using my phone, prob about 500 texts a day (about half dictated, love that feature), phone calls up to 30-40 min a few times a day, leaving wifi on at home and at work, and I am using about 60 percent of my battery each day.
I was able to go fri to sun before first charge died, and now I am currently waiting for the final 1 percent to fall so I can do the 2nd and final caliberation til next month.
I find the battery life to be quite better than my 3GS and although I have no iphone 4 to compare it to, I think my battery life is quite fine. I am averaging under moderate use a day and a half of use, whereas heavy use like today and i went 1 day, which is all I want, to make it from wall socket charge through the day to another charge...
I do have wifi sync turned off, I was noticing it was trying to sync every now and then even though it wasn't plugged it, and I have location services turned off for apps that don't need it.
The notification center has location services for weather, so every check activates that location GPS icon, can drain the battery...i turned it off, and added the 5 cities I am always in, and if I need to check another cities weather, I ask Siri or I can open my Weather Channel or Accuweather App...