I'm running Yosemite on an Early-2011 15-inch MacBook Pro, and the fonts look a tad bit sharper than Mavericks, but I suspect this will be refined in a later build.
The quality of the fonts in menus it highly dependent on the color of your background. Light background looks great. Dark background looks terrible. Steve was a major font quality person and I know he would not be happy with Yosemite's fonts. Hopefully it improves before release.