I have owned 2 iPhones (a 3G and a 4) and was getting a bit bored of ownership and was also tiring a little regarding how locked down and inflexible they sometimes were (for my own personal needs - although perhaps not for everyone).
Also - I found it quite annoying as an iPhone 3G owner when every software update just seemed to make it slower and slower...
This year I fancied a change and moved to a Samsung Galaxy S2 (SGS2)and I get on really well with it:
Now, if I want to copy music on to it - I just drag and drop from my PC with windows explorer, if I want to watch a video - again it's just drag, drop and off I go. I can set any of my music as my ring tone (something neither of my iPhones did previously). I have a choice of getting apps via Android Market, Samsung and Vodafone straight out of the box. If I want to use my phone as a wireless router it takes just a few clicks and off I go (on my iPhone they wanted to charge me a monthly tethering charge too?).
The screen also looks great - it is big, bright and colourful - and when it displays black it is very very black, so anything on a dark background looks really impressive. Internet access is super quick. There are tons of free apps out there - I haven't found anything I need yet that I have had to pay for! All in all, for me personally, it has proven to be a great phone.
Are there any down sides? Well, the older versions of Android had a few bugs with this phone - most of which have been sorted in the latest Android release that Vodafone have put round. I think the only thing that can be a little annoying sometimes is that the volume can be quite low compared to other phones - although even this seems to have improved with the latest Android release that Vodafone offer.
The SGS2 is slimmer and lighter in feel than the iPhone 4 (although some people say it feels cheaper too) and it has a much bigger screen area (although at a lower resolution than an iPhone). I find text is much easier to read on the SGS2 screen (although perhaps my eyesight is not the greatest - the iPhone 4 text was much clearer when it was really small!)
I am not an Apple hater - I own iPods, MacBooks and an iPad and I did like my iPhone 4 - but I do think that my own user experince with Android running on the SGS2 has been better - for my use it has proven to be a more flexible phone - and it is currently also a £100 or so cheaper than the Apple equivalent...
I am not trying to start an Android vs Apple debate - it is just that for me to provide feedback, it is easier to compare my SGS2 with an iPhone 4 as that was the last phone I owned before I switched
