I used to use Aperture but recently switched to Lightroom for 3 reasons: adjustment brush, before/after side by side comparison, and file organization.  Well I didn't really switch since it's close to impossible to move all the raw files with adjustments over, so I just processed recent works on LR.

Aperture 3.0, released today, adds adjustment brush, before/after side by side comparison plus tons more new features, including face recognition, map tag, and slideshows. They have tons of new features under Organization although I can't tell if they fixed what I hated from the descriptions.

I'm really excited and can't wait to test it out. If I like it then I can go back to aperture and not having to worry about transferring files.

I don't know how many are Mac users here.  But if any of you is interested I'll post a review here if you're still on the fence when it comes to LR or aperture.

I've never used aperture but I do have a MAC, I'd be interested in hearing your review. I have lightroom 3 and love it, use it for all my post processing. I use photoshop too if needed:)

If you always use LR I guess there's very little incentive for you to switch, unless you absolutely love the seamless integration with iPhoto.  Oh did I say it even processes videos now?

I forgot to add that you can try it for 30 days for free here http://www.apple.com/aperture/trial/

I finally got around to install and use aperture 3.  I'm disappointed.

To give you a little more background, I was a long time user of Aperture since Aperture 1.  A few months ago I discovered the "skin smoothing" brush in Lightroom, and decided to switch to lightroom.  So it's only been a few months between Lightroom 2 and Aperture 3.

Most photographers dislike Aperture for its "unintuitive" user interface.  I never had a problem with that, since I started with Aperture.  In fact, when I first used lightroom, I found it somewhat frustrating too because the buttons were not where I thought they'd be.  But to me that's just a given when switching programs.  I quickly got used to the UI and memorized all the shortcut keys in a few days.

Last night when I used Aperture again, I was shocked how clunky the user interface seemed to me.  Keep in mind that I used Aperture way longer than Lightroom.  I almost wanted to give up in the first 5 minutes but I forced myself to keep trying. That's when I realized, dang, Aperture UI is really unintuitive!  If you stop using it for a few months it'll turn into a total stranger to you lol!

The really annoying parts are 

-They still don't have Before and After comparison! This drives me BANANAS!

-Yeah they added adjustment brushes alright, but it's not as easy to get to as on Lightroom UI.  I haven't tried the brushes yet so I'll save that for later.

-Number of Presets can't compare with Lightroom 3, and most of the presets... suck!

-All the controls are on the left side.  So if I want to add tags or keywords to a photo that i'm working on, I need to flip to the metadata tab.  I much prefer Lightroom's two control panels-- one on left and adjustments on right.

-Finally, it's not as fast as Lightroom.  That just blows my mind.  I need to wait about a second to see the preview of the sharpening effect. Aperture is supposed to be faster than lightroom on Mac because it's Apple's baby.  Speechless...