I've been a loyal and passionate Apple user since we got an Apple IIc back in the '80s, and I even waited in line with @cw for the first iPhone when it was released. However, I switched to Android a few years ago, for the following reasons:
• Google Voice — Apple had rejected the iOS Google Voice app, so Android was the only way to use it natively.
• Unlocking — Apple wasn't yet selling unlocked iPhones, and I wanted to be able to freely move to different carriers both in the US and when traveling internationally (the Google feature phones are unlocked by default).
• Tethering — though iOS supported tethering, the Carriers gate its availability and charge extra for it, which is preposterous (bits are bits), while unrestricted tethering has been in Android for a long time. (I use prepaid data service for this)
• Cloud sync — Android's Google Account integration works directly from the cloud (email, contacts, calendar, docs, etc.), while iOS back then made you go through desktop iTunes for these things.
• Misc policy things — for example Apple's nonsensical in-app payment policies (which prevent apps from using their own payment means), their draconian approval process and content policies, etc.
• Hackability — without rooting, and while staying on the official OS upgrade trunk, Android lets you do things like make free calls with Google Talk, use a better Intent UI, dim the screen beyond the default settings for better nighttime reading, scrobble to LastFM, etc.
• Intents — probably my favorite Android OS feature, Intents let apps communicate with each other. For example, I can email something to myself with a single tap, post a link to a GroupMe group without copying/pasting, etc. Intents are great, and they're also coming to the web. So stoked for this.
• Other misc UI things — system widgets (for turning things on and off like the LED flashlight, wifi hotspot and airplane mode), app widgets for things like playing/pausing audiobooks and music, etc.
Tom articulates a few more reasons as well.
That said, Android definitely doesn't provide nearly as refined a UX as Apple has created with iOS, which I do miss. Apple's stuff is so beautiful that you just want to use it for the sake of using it, which I definitely don't feel in Android-land. But ultimately the choice is about tradeoffs, and for the time being, I'm going with better functionality (for my needs) while sacrificing a bit of form.
Update, 2012-09-18 — Nick mentioned two other great Android features I neglected to include in my list, Google Maps turn-by-turn navigation (coming in iOS6), and Google Now, which is surprisingly useful. These days most of these things are possible in iOS — Google Voice, unlocked iPhones, cloud sync — but I don't anticipate rootless hackability coming to iOS anytime soon.
10 hours ago