I'm impressed. I tried Adobe - Photoshop Lightroom (OS X) when it was in beta and I wasn't that keen on it, but it's come a long way. Editing tasks are quite fast on my G5 iMac, certainly faster than Aperture, but even quicker than iPhoto. You can actually edit a date in Lightroom, which you can't do in Aperture (still). (Ok, so there are bugs in dates prior to 1600 or so ...)
The relatively small number of keyboard shortcuts are just right to make quick work of editing. I quickly got the hang of ~, R, G, Cmd-U, etc. The auto-correction and several tools for saving and applying sets of editing operations worked quickly and well.
I don't see the powerful querying and image management tools I used during my Aperture trial however. They may be there, I haven't finished looking at the app yet.
If Adobe had decided to support migrating from iPhoto I'd be strongly tempted to buy it. Of course they didn't. Lightroom is not a good option for anyone with a great deal of metada in iPhoto, but it may be the best bet for just about everyone else. This 1.0 release of Lightroom seems better put together than Aperture 1.52.
Did I mention the user interface is clear and readable -- unlike Aperture's bizarre non-Apple GUI?
Apple needs to get in gear. They're already losing a race that's barely started.
Update 3/8/07: I've found my first nasty bug. On occasion Lightroom reports it cannot import an image. It may even say it can't be read. On a retry Lightroom reads it. Image Capture, iPhoto, etc have no problem with the same image.