I got my extended battery today and was suprised at how much bulk it DOESNT add to the phone. I had a thunderbolt before and I bought the extended battery when I got the phone. I took the battery back the next day because it was just comically huge and made the phone uncomfortably large and heavy. Here are some pics comparing a stock Thunderbolt to the Droid Bionic with extended battery. Also pics of the stock Thunderbolt vs Thunderbolt with extended battery. It should give you an idea of how big the Bionic extended battery is. IMO, it is definitely liveable.
Thunderbolt (Stock) vs Bionic w/ Ext Batt.
Thunderbolt (Stock) vs Thunderbolt w/ Ext Batt.