Yes you should be able to downgrade to OS 5 if you do not like OS6.
You can find the instructions on the blackberryFAQ page under "Applications and Operating System FAQ's"
But why would you want to?
I originally had the same plan.. I've been running OS 6 since Dec 22. And my original thought was "well this is slick, but is it worth the upgrade?"
Well now I have to say yes. This may be only anecdotal but I find OS 6 a bit faster than OS5. And even though my free memory was cut in half I have not had any issues with this. I've installed 20 apps and still have 51MB free. almost the same as a fresh install.
My biggest complaint is the lack of themes, but this is not a reason to downgrade. I'm sure the themes will come in time.