I have a Canon 20D and a couple of 30D cameras. I have upgraded the firmware in each as the software has been released. For the most part, I've not seen any impact either positive or negative.
The released software should come with a statement of what is changed and the purpose behind the software. If you are, in any way, concerned then I would read the release statement and decide from that.
Some times the firmware fixes things that I never use and thus, I never noticed the problems it supposedly fixed. Othertimes, the firmware fixes somethingthat is incredibly annoying, such as when the camera decides to lock up (D300) at random times. You can bet the moment I found they had released a fix for that, it was on my camera!
Definately read the release statement and do what Scott said.
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The released software should come with a statement of what is changed and the purpose behind the software. If you are, in any way, concerned then I would read the release statement and decide from that.
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Definately read the release statement and do what Scott said.