Recently my husband and I decided to make the switch from an iPhone to the Android-powered HTC Evo. There were two main reasons. Our iPhones were 3G, and while they were awesome and ran at top speeds when we first got them over a year ago, since then they've slowed down noticeably--we both wanted a new phone, iPhone or not, and were frustrated at having to wait until our contract was up.
The other reason was cost. AT&T's switch to tiered data plans this summer didn't affect us on our current contract, but if we wanted to upgrade our phones and sign a new contract in the future, it would be something we'd have to face. Plus, let's face it--iPhones are great, but they're not cheap.
We switched to Sprint and chose the HTC Evo, which appears to be most like iPhone, at least in appearance. It runs Android 2.2 and has a touch screen keyboard, comes with an 8 GB SD card, plays music, internet, email, etc.
So far, there are a lot of good differences. No customization options are ever 100% complete, but compared to the iPhone, it sure feels like it. Customize your ringtone, text tone, and email tone for free and without hacking the phone. The home screen has more options too--I was a little confused at first because on the iPhone, the home screen is basically your place to launch apps, and you can possibly incorporate a neat background or some effects. But on Android, you have a separate button where you go to open your list of apps--so the homescreen becomes a place where you can pick and choose which apps you want quickest access to. Both Android and HTC have "widgets," which gives you more customization options--weather, email, calendar, battery life, and any widgets that come built-in with your apps. (My To-Do List app, Remember the Milk, comes with a widget that will display ten of my to-dos.) The home screen is actually made up of seven different windows, and based on the sizes of the widgets you choose and the number of apps you choose, the combinations are endless. You can also put apps in folders (much like the iPhone iOS 4.0 and up).
Something else that really surprised me--you can expose your home screens! This is a feature on my Mac that I LOVE, and I'm surprised to see Android incorporate before Apple (unless I missed something?). Hit the home screen button twice and voila--see all seven of your home screens at once.
There are some noticeable downsides as well, although whether they are downsides or just differences is up to debate. For one, your apps run all the time. Open an app and then exit to the home screen, and it's still going. Other apps may be updating in the background. This will drain battery on any phone, and the Evo is known for its short battery life. A task-killer app is a must. (I use Advanced Task Killer.) What's nice is that a task-killer app will let you go to just one place to kill multiple apps at once--it's just a matter of remembering to do it.
Something else that took me by surprise is that there are so many phones running Android out there. This is obvious, but you don't really think about it until you start reading app reviews--some apps may work great on one model that runs Android, but the app might not be formatted for other models and can cause crashes. What works on your phone might not work on someone else's, and vice versa. This is a different mindset for me, coming from the iPhone where things might or might not be formatted for an iOS update--thats it.
A final thing that struck me as weird is the idea that my phone can now get viruses if I'm not careful. The iPhone was my first smart phone, and I had never worried about it before. As far as I know phone viruses aren't widespread yet, and there's anti-virus software you can get for your phone. But again, it's something I had never thought about before.
In my opinion, both operating systems are great and have their strengths. I do miss my iPhone sometimes, and would I buy one in the future? Absolutely. But there are some great phones out there for a much lower cost, and right now it's more important to me to explore those options. I love the iPhone--but I would suggest picking a phone based on more than whether or not it's an iPhone. Do some research, evaluate your phone needs/wants, and you may end up saving money.