So I had installed the Battery left widget and used it with both batteries now. In terms of my general usage I imagine it is fairly standard and average for most of us. Lots of e-mail, occasional web surfing, maybe 15 minutes total throughout the day of a game or two. I would say I am a heavy-moderate user. Oh, and Pandora while driving places...
Every morning my battery left widget would predict the batter would last until about Midnight that night. Now, with the extend battery it predicts it will last until 4:30 AM... So with my style use it gets me an extra 4.5 hours. I've found it totally worth it.
Just in contrast though, my wife is a very light user. She tends to only make occasional calls, e-mail and text a few times a day. She still has the original battery and I saw her widget yesterday day and when she unplugs in the morning it says that it would die around noon the next day! So, for her, the extend would probably mean a solid two days of use.