The practical ones:
- I'm a heavy Gmail user, and Android offers up-to-the-second push Gmail, and a Gmail interface that far surpasses Mail on the iPhone. Reading, searching, labeling, and otherwise processing my email is the most important thing I need to do on my smartphone besides make calls, and it's simply easier in Android.
- Android lets me manage my contacts in one place: Gmail, and syncs them automatically to my phone. No more local address book!
- Android is better than the iPhone software in a few ways: it's way more customizable, it offers copy and paste, the pull-down "window shade" is a better notification mechanism, and in general it gives you that feeling that you're in control of every setting.
- Android doesn't run Safari, but it will likely run Chrome someday.
- I love the trackball on the G1 for scrolling and clicking. From an economy-of-motion standpoint, it's a way more efficient way to interact with the device than swiping and tapping the screen. The snap-out keyboard is sweet, too.
The philosophical reasons:
- Android is open source.
- Because it's open source, many of my favorite open source apps work with it and not the iPhone yet, like KeePass, for example. (See KeePassDroid.)
- Android's apps are written in Java, and as a Java developer, this delights me.
- Android doesn't tie me to iTunes, which is a fine piece of software, but is just a little too bossy and proprietary-like for my taste.
Obviously, your mileage may vary. But there is a misconception that the iPhone is a superior smartphone, and that's not necessarily true. What's true: The iPhone software has been out longer than Android, so it may be more fully-baked (though apps and Safari crash on my iPhone regularly, more so in the 2.0 "upgrade"). The iPhone has a much larger market share. There are probably more apps for the iPhone than for Android, but the Android Market is well-stocked, and I have had little trouble finding the stuff I need. The only iPhone thing I truly miss is its built-in visual voicemail, but I've installed PF Voicemail+ on Android to get that same functionality. Not being a heavy voicemail user, this isn't that big of a deal to me (though it may be for you). Update: I also miss the excellent Evernote iPhone app, and have my fingers crossed in hope an Evernote equivalent will happen soon (or at least a mobile Evernote site that works well in a mobile browser).
Most likely I'll remain in the minority as a G1 owner, but you know what? In a sea of iPhone-toters, I'll enjoy actually "thinking different."