I bought aperture since it was the only finished product of its time at the time. I didn't know if I would like LR since the beta was a bit buggy. Once it came out I gave it a shot it is no where near as buggy as the beta of course but it still wasn't as fluid and easy to use as LR. With LR you have to learn how the app wants to work.... with Aperture you make the app work for you.
Aperture handles files better in my opinion whether you work with the files in the combined folder or through your finder. With LR you only have one choice and that is through the finder. Aperture organizes files, folders, projects, and so forth worlds better than LR and LR couldn't utilize my screen space efficiently... it wasted a lot of space making my images and thumbnails smaller. LR didn't have the ability to make books, light tables, and the loop which I use constantly, and LR doesn't allow me sync my entire photo library with the rest of my Mac apps like iWork, iLife and Final Cut Pro 2.
Image editing on both apps was negligible so I stayed with the big guy. LR was good and it great for people that don't have a machine that can run Aperture but when it came down to what both did and what advantages each had over the other Aperture just has more in the bag... it has everything LR has and more... and does it better without hindering my workflow... since my WF changes depending on my shooting environment and how I feel.
As for my own opinions... I feel that LR's UI is rigid and boxy with the filmstrip at the bottom never being able to move. In Aperture I can look at it in grid view with large icons or filmstrip or hide it entirely. Full screen with two monitors is beautiful while LR can only use one. Aperture allows me to save every single thing, change, action, caption, export option, adjustment, preference you name it as something that I can recall and easily find when I need it. LR did have a wonderful way of organizing with flagged images and labels, but I found a way to do that with keywords in Aperture.
I did think about making a switch to LR once I got my hands on both but after I used them both for a few weeks I found myself getting things done in Aperture... while in LR I found myself trying very hard to remember where and how I do things in the different modules and how to do things like Aperture.
I have my library of 10,000+ images on drive on my G5 and link both my MacBook Pro library and G5 library to that drive. When I need to sync projects I export from one computer and import into another. Now that I have my workflow down and not locked into the program... I am going to stick with Aperture for a good while. I still keep LR around to show fellow colleagues and college students the benefits, pluses and minuses of both. I leave my bias aside because I'd rather have someone buy Aperture because they like it and not because I told them to.
When I need to look at images quickly and without loading a massive amount of preview from LR or Aperture I use Bridge CS3 which is the best image browser I have ever used... better than the $$$$ PhotoMechanic. With Bridge CS3, Aperture, and Photoshop CS3 (possible NX in the near future) there was just no room and now where to stick LR.