I've been using the new (1.22.1) release candidate for a few days now, and it's an amazing memory hog, at least on Vista. After about an hour and a half this morning, Vista reported memory use for SecondLife.exe of close to 700 mb. A similar test this afternoon yielded about 500 mb. My system was starting to slow way down.
I have also experienced a higher-than-normal rate of crashes with the new RC, as have others.
One hopes that those kinks get ironed out before we're compelled to use the new viewer.