A powerful and somewhat elegant (and sharp) UI built on a foundation of flimsy playing cards. Verdict: weak sauce.
Yes, you read right. Apple makes it, and I think it sucks. Don't get me wrong, iPhoto sucks worse, but Aperture still sucks pretty hard.
My longest-standing complaint and something that I doubt Apple will ever change. It's the primary thing that has always kept me away. At the heart of any such program is its organization and management capabilities. Aperture allows you the use of only a virtual system of containers (you can never get a representation of your file system in the UI). To make matters worse, you cannot organize your photos to mimic the file system either. That's because Apple uses three incompatible "containers" to manage photos. Projects, Albums and Folders. You can nest folders, but they can't contain photos. You can put photos in projects, but they can't be nested. You can put photos in albums, but only if they're already in a project.
This is complete lunacy and makes what could be so simple, so very convoluted and unworkable. All they needed was one type of container and then let you do whatever the hell you want with it. Like Lightroom does.
You can't set a preference to always import by reference (or copy and then reference). It supports this feature, but if you forget to use the right setting or you happen to drag and drop, you'll end up with content in the Aperture library instead.
You can't import, sub-folders through the import UI. Fail in a big way. So if you've used any other software to manage your photos, or even if you've simply kept them organized in folders yourself, you're going to rip your hair out trying to get them into Aperture because the Import feature requires you to drill down to the last folder containing images. That means one folder at a time. I have hundreds.
Tying some of these together... On import you can have Aperture copy your photos to a specific location and then create sub-folders by year/month/day. Now let's say you accidentally trash your Aperture library and want to put all these photos back into Aperture? Oops, we're back at the issue where you can't import sun-folders and have to go through them all one at a time.
Now, Adobe has a crap-looking UI, but at least they got the very basic level of management and organization done right.
At this point I can only see using Aperture for doing one-off or sngle-project editing. It can't serve as a repository for someone with a large photo collection, IMO.