I have the Otterbox commuter and would recommend. It adds bulk for sure but not tons. My BB has fallen off of the dash of my car striking a cup holder/center console on the way down and no damage at all. Not sure what WOULD have happened but thankfully didnt' find out.
I have the Defender and Commuter case. I love both of them. I use defender when I am going to be places I might rough it up. For normal activity I use the commuter case. I have the seidio extended battery on my phone and it works great with the commuter case. Either case has lots of protection.
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I used the Otterbox Commuter on my Bold 9700 and I loved it! It was great feel and protection. I also used the Seidio Rugged and it was a lot beefier and is compared to the Otterbox Defender.
Commuter all the way! I didn't have much choice because of the extended battery. The mod on this case worked great with the bigger battery and I couldn't be ahppier with it.