If you're dying to get your hands on an early version of Photos for Mac, Apple just released the first public beta of Yosemite 10.10.3.
The brand new iCloud-based Photos app is 10.10.3's biggest new feature, replacing both Aperture and iPhoto. The upgrade also includes the widely discussed diverse emojis and easier login for Google users with two-factor authentication turned on. To install the public beta, you need to be a registered beta user. Then click on over to the Mac App Store's Updates icon, where the 10.10.3 beta will be ready to download.
Before you upgrade, remember that beta software isn't completely polished and may be buggy. Follow Macworld's tips for prepping your iPhoto library for the Photos transition, and then read our Q&As here and here for answers to common Photos-related questions.