Tried both, use Aperture, largely for the same reasons as other Aperture-lovers: more freeform interface, better stacks, and in particular multi-monitor support and a better multiple computer workflow (since I'm often handling images on both laptop and desktop). There are, of course, things LR does better (love the B&W conversion and the indexing by lens, camera, etc.).
I can't believe the latest update didn't provide even rudimentary multi-monitor support. Would like to hear what LR's new multi-computer workflow options are like and how people find them....
I have recently switched to Aperture from Lightroom.
Two things Aperture does sinificantly better (IMO) are actual photo storage management, and "lights out" viewing and editing. I like to have all my raw files stored in a central location (Aperture library), and I like not caring about what the software does with the files. When I was using Lightroom, my Pictures folder was a mess - and I had to do my backups to the external drive with Chronosync. Not anymore. Aperture's Vault feature is amazing.
With version 1.5 you can also store files in your own folders, just like with Lightroom... I just don't know why would you want to.
I like the Aperture raw converter resulst to better than Lightroom's (I use an Oly E1).
The thing that Aperture does dramatically worse than Lightroom is dust patching. By default the tool just blurs the area (not very helpful). You can switch the tool into Lightroom-like "stamp" mode. However, unlike Lightroom's implementation, the tool does not always find a good place to stamp from. If your dust spack is on the edge of the photo, the "rubber stamp" may be placed outside the frame. What is worse, patching is excruciatingly slow. If you patch 10-15 spots on an image, the Loupe tool becomdes almost unusable due to lag.
I have recently switched to Aperture from Lightroom.
Out of curiosity, how did you manage migration of your library? I'm about to do the same... partly for speed, partly because I really like the develop module, and partly because I may switch back to windows *gasp* At least until Apple releases a mid range monitorless mac.
It's pretty straight forward if you already have your pictures in a folder structure. If they're in the Aperture library it's harder because you need to export everything and then organise it, then import into Lightroom.
Of course you'll lose your edits and your tags(although I believe there's a way to save them) but the rest should be pretty straight-forward.