I must be one of the very few Aperture users who actually like it. Yes it's very limiting and doesn't offer anywhere near the same capabilities as Aperture did and no it won't be the only software you'll need if you want professional photo editing. But: iCloud integration and the Library appear to be fantastic. It carried over all my Aperture adjustments very well, although I suspect they're just full-size previews, and it does VERY well what I need it to do for most projects: RAW Conversion, Exposure, Highlights, Shadows, Levels, WB...
If my processing needs go beyond that, there's still Photoshop. And let's be honest, neither Lightroom nor Capture One are proper replacements for Photoshop most of the time.
Also, the optimist within me hopes that NIK, VSCO et al. will add some functionality to Photos in the future.