Good decision, you won't regret the 8900. If you want to use that pouch it came with just put your phone into standby and then put the phone in screen first so then it doesn't go into holster mode. The holster/lock mode will drain the battery faster than standby.
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I read on a previous thread not to use the sleeve it comes with because it's too tight, still seems to be with my new 8900. I have mine in a Incipio hard case, and then use my sleeve from my 8310. Fits perfectly and then put it in standby, works well.