Why did you switch? Lack of faith in Apple to maintain desktop software in an iOS world?
No, not really. It mostly had to do with the way Aperture manages files. I very often need access to my original RAW files for use in other applications (such as Photoshop) and Aperture makes it very difficult to get to them. Lightroom also has a significantly wider range of editing tools, and I much prefer the UI. Adobe just knows how to make better imaging applications than Apple. Aperture is more geared towards the advanced amateur and Lightroom is a full fledged pro app, in my opinion.
It may also be worth noting that I'm a photography student and all of my professors use Lightroom and it is installed on our of our computers.
Also: I wanted to switch to lightroom ever since I first used it when 3 came out, but absolutely dreaded the process of migrating my library. Finally last summer I had had enough with Aperture, Lightroom 4 had just come out, I had a few days with nothing to do, and I decided to just do it once and for all. I didn't migrate the entire library, only the important stuff, but the whole process (including converting everything to DNG) took about 24 hours total. And as you can see I'm using a pretty fast Mac Pro with eSATA drives