I've just uninstalled the new Yahoo! Messenger 8 Beta from my machine. It kept crashing spontaneously and when it did work, it comsumed whopping 42MB of my RAM and considerable CPU percentage (~30%)
I've reinstalled the older Yahoo! Messenger 7.5 (get it here if you're not satisifed with 8 Beta) which "only" takes up ~30MB of RAM and is stable.
I can't wait for Windows Live Messenger to support Yahoo! Messenger users already, so I can dump Yahoo! Messenger completely.