Just managed to install 10.10.3 on my old 2008 iMac so I can try out Photos. I upgraded to a 2011 27" last year, so I can afford to experiment on the old one. This was by far the most troubled upgrade of OSX I have tried, but that's another story, it is up and running smoothly now, and Photos is importing an old Aperture library for me to test on. This one won't be connected to iCloud, as I don't want gigabytes of Raw photos uploaded. I will later create another empty iCloud library to sync all my phone and iPad photos across.
Like a lot of photographers, I was very weary about Aperture's future some time ago with the lack of major upgrades, with various rumours about them working on a major overhaul the way they have before, namely FCPX. It was obvious they wanted better integration and syncing with their iOS devices, which could be a killer feature for iPads for creative professionals, in particular photographers. Artists are still waiting on an iPad Pro with stylus support, at least it seems they will get force touch.
Alas, no Aperture-like iOS app synced with a new polished iCloud-integrated Aperture; instead we get a very consumer-oriented Photos across the line, great for the mass of iPhone casual snappers. Aperture is truly dead, with no hint or reassurance something is coming.
I am yet to try Photos out for myself as it is busy importing a large library as I type, but going by the reviews from a photographer's perspective, it is not meant for any serious work, no prospect of a surprise Aperture Pro. A hint of that is for instance when it imports ratings and colour labels, they are simply converted to keywords. From hereon in, it is Hearts or nothing. So no proper rating and labelling system is planned for the near future given they don't preserve it behind the scenes. Nevermind all the other missing features, if they don't even make an attempt to preserve certain existing functions photographers rely on, it doesn't bode well.
So basically I hope to use it to manage my i-device photo albums, and hopefully it can be integrated into my social media workflow. Currently I use Dropbox, and more recently, Photosync, as a way to get photos from Aperture onto my iPad to be further edited for Instagram and Facebook.
The troubled Yosemite upgrade which required a reinstall from my backup and starting over doesn't fill me with confidence for my main production machine; Aperture will remain my main photo app for the foreseeable future, after which maybe Capture One will be my mainstay, perhaps with some Photos App for exported finals for Instagram and Flickr, as I enjoy working with various iOS apps.
There is nothing to truly replace Aperture, certainly Lightroom has evolved much better editing tools, but its workflow and metadata editing sucks. Capture One is the closest so far, and seems to be moving in the right direction, but also exceedingly expensive. Maybe by the time I'm ready to move, it will have even better Aperture import. As it is, it does a better job than Lightroom in preserving what it can from Aperture.
I will assume for now the unavailability of Aperture for upgrade is a temporary glitch, as it needs upgrading before it can run on Yosemite. They have clearly said, and others have confirmed, that all previously acquired or installed apps will continue to be available, just not for new purchases. I feel that ever since they announced Aperture's demise, they should have made Aperture available for free. I'm not sure if they made it clear on the store that it had been deprecated.