I have both batteries on my Bionic. I unplug the phone at 6:30am and by 10:00pm the regular battery is around 15%. Yesterday I used the extended battery for the first time and had 50% after 15 hours.
I typically make about 10 calls a day, play Angry Birds for an hour, read e-mail all day long (I get about 100 messages a day), and read stuff on the internet browser several times a day. My 4G signal goes in and out all day (3G downstairs/4G signal on the second floor), so my data is a combination of WiFi and LTE.
This is a very unscientific study, but the battery life is good enough for me. Furthermore, the above results were acheived without plugging in the phone all day, and since I have a mini USB at my desk, and one in my car, I will probably be charging the phone mid-day on a normal basis.
I think I will mostly use the standard battery since it is lighter and thinner and it's good enough for me. But it's nice to have the extended battery for trips - especially since I got it for $29.