I'm on my 4th (5th if you count the temporary Galaxy Mini I was using a few weeks ago
) android now and would never consider an iPhone.
I've owned the original HTC Desire, Desire HD, Galaxy S2, (Galaxy Mini), and now a Galaxy Note.. all fantastic (in their day) phones and the GS2 would still be (imo) a very good phone even now.
As others have said it's the ability to change practically everything about it that appeals to me (yes I work in IT, but my nerdy days are more-or-less behind me now
Don't like the interface? No problem.. change it
Don't like the keyboard? No problem.. change it
Crap wireless reception? No problem.. change the kernel/radio
Sick of ads in apps/games.. No problem.. get an ad blocker
Sick of the phone itself? Want a bigger screen/more features but keep your apps and launcher etc.. No problem, there's an Android phone in every size/spec/price point you can think of
etc etc etc...
Now admittedly you will only get the most out of the phone by rooting it and installing custom ROMs - which does require patience, the ability to follow instructions
, and some trial and error .. but once you do that, you're sorted!
I currently run KingDroid 3 ICS ROM on my Note with the Launcher 7 Windows Phone-style interface (suits my stubby fingers better!) and I love it.. the bigger size (5.3" screen) is something you get used to after a day and I can already tell it'll be hard if I go back to anything smaller in the future.
For balance though.. I also have a Windows Phone for work and (as above) I like the launcher, but the lack of customisation (I hate the keyboard for example) and basic settings that are missing from it are a massive downfall of the phone in my opinion. WP8 looks better but the fact that it's not going to be backwards compatible (I know there's a 7.8 update planned that will bring the interface to the older handsets but I mean more the new features like Bitlocker and so on that won't work) is going to be a massive problem for them imo.
I've also played with the iPhone 4 a fair bit and like the Windows version it's even more restrictive. iTunes sucks. It's tiny, there's the infamous wi-fi reception issue and compared to Android and even MS's interface it feels dated with rows of static icons.
But definitely try all of them out and see which you prefer yourself.