I started with Lightroom 1. When Aperture was released I wanted very much to use it, as it "thinks" more like I do. Plus the OS/iLife integration is top notch. Trouble was it was a slow, awful mess. When Aperture 3.0 was released I tried again. Slow, buggy, crashed - so I stuck with Lightroom.
After seeing the $79 price for Aperture in the App Store, I've been trying it (3.1.1) again. Much improved. It hasn't crashed feels as fast as Lightroom. I'm running a 27" iMac with 12GB of RAM, so mileage may vary.
I really like Aperture's DAM tools. They can be confusing at first, but are quite powerful. I manage 90% film scans in it, so absolute RAW conversion quality is less important to me. Many folks feel they're close enough, with a slight edge going to Lightroom. The editing tools are similar. Much depends on how they feel to individual users. If I had to compare off the top of my head...
I like Aperture for...
* OS/iLife/iPad integration
* Books/Slideshows
* DAM capabilities
* The UI more naturally fits my way of thinking
* Price
I like Lightroom for...
* Ecosystem (Tons of presets, plugins, and resources)
* Adobe tools integration (Photoshop)
* Crop and Rotate with same tool (I love this, not everyone agrees)
* Exporting options
* Publishing Services
* Noise reduction in LR3 is terrific
Lots of much deeper comparisons out there, but those are the pros of each for me.
Now that Aperture is usable for me again, and cheap, I've started a new library and am importing everything into it this year (until something breaks) and see how it goes. I'm even letting Aperture handle the files for me in the Library package, which goes against my grain a bit. Guess I'm all in.