buy Aperture at the moment?

As you wish. Aperture is going to be discontinued next year, so I would not recommend you to use it. Instead of Aperture, you have great photo editing apps such as Pixelmator (available in the Mac App Store). You can also wait for Photos, but do not expect as many features as with Aperture (Photos is going to unify Aperture and iPhoto).

Aperture, LightRoom, Capture One Pro are all playing in the same league as professional applications. Of all three, Aperture s the one, that is best integrated into the Apple environment. It has a very intuitive User interface, and is a joy to use, once you have learned the typical workflows and understand the concepts. It is a great tool to have and less expensive than the competing applications.

Apple announced that the current photo applications will no longer be developed and replaced by a new "Photos" app.

See: Aperture is Dead. Long Live Photos! | ApertureExpert

The problem is, we do not know yet, what Photos will be like. We have not seen more than screenshot, and that is not much to go by.

If you buy Aperture now, you can be sure to be able to use it during the life cycle of Yosemite, maybe longer. You could start with iPhoto, that comes free with every Mac, and test, if it will suffice or you require advanced features. Upgrading from iPhoto to Aperture is easy, since both applications are using the same library format.

Migrating from iPhoto to Photos will also supported, and if you find, that photos does not suffice for your needs, you could still upgrade to Aperture or one of its competitors.

Aperture is a waste of money at this point. It is only barely cheaper than many competitors (especially since many are running sales now) for a product that is going nowhere but the grave. And obviously some of us disagree about it's ease of use and integration with other software you may use. Maybe Photos will be better; we don't know when it's gonna show up or what it will be beyond a brief preview, so if you need something and can't wait, try others.

It's a matter of personal preferences, what you need, and how you like your images to look. But even before Apple axed Aperture it had gone a very very long time without anything but minor upgrades. Meanwhile, the competitors have been improving.

Also, you can't try Aperture. But you can try demos of all the competing products. Some favorites besides Lightroom/Photoshop (pretty much the professional standard right now is, and has been, Photoshop) are Capture One Pro (now supporting external editing with software like the Nik/Google products, Photo Mechanic (a favorite for news and sports photographers), DarkTable and Lightzone (open source alternatives), Corel AfterShot Pro, and others. Some like DxO OpticsPro use photo browsers instead of the digital asset management scheme of Aperture and Lightroom, and some prefer that model. DxO excels in RAW processing, as does Capture One, if RAW is important to you. Others are more Photoshop alternatives for image retouching and manipulation, like Pixelmator, Photoshop Elements, Acorn, GIMP, and others. Another worth looking at (again, we don't know your needs) is Graphic Converter, which has a browser, and editor, and great tools for metadata editing.