OS upgrades usually fix issues of the previous OS version. OS upgrades usually include new features such as threaded SMS. Added memory, longer battery life and quicker device performance are also an advantage.
However, sometimes the new OS upgrades come with bugs of their own to be fixed. Another disadvantage is that if improperly installed, you can "brick" your BlackBerry, making it useless until you go through the steps to reload your software. This can be time consuming and daunting for a new BlackBerry user.
You can also upgrade your OS right from your device OTA (over the air) once officially released by your carrier.
You can also install leaked or beta OS's once you feel confident of your abilities. i'll let others speak to this as i always stick with carrier released official OS's.
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