Having had the 3GS for nearly 2 years I was ready for an upgrade and as soon as I was able to do so, I upgraded to the 4S (the latest iPhone model at the time I upgraded back in March 2012).

Look & Feel
First of all its a lot more rectangular than the 3GS with rounded edges which I really like, it looks a lot more upmarket and stylish in my opinion. I like the way it fits in my hand better. The size is perfect, it's not too big but its big enough so you can see the screen well.

Screen
The screen is large and bright, very easy to see and understand, the colour quality is fantastic with bright bold colours that stand out well. You can very easilly adjust the brightness & contrast settings to help make it easier for you. I have mine on a bright setting as its the best quality I prefer and find easier to see.

Ease Of Use
The iPhone 4s uses the same layout as the previous iPhones with the very easy to use interface, on the main screen you can lock it and to unlock its so simple you just swipe your finger after hitting the main button. The "apps" are shown on various screens and I like how easy it is to flick through by just a swipe of your finger on the screen, this does however make the screen get dirty with finger marks but you can buy screen protectors to combat this.

The Camera
The camera is the best I have seen on a phone before (that is before the 5 came out), its easy to use and I like having a camera on the front and back of the phone, you can use it as a normal camera on the back of the phone but it also enables video calling using the camera on the front so you can see who your talking to. Great for me as I work abroad during the Summer I find this nice to use and see friends & family at home. The quality of the photos is rather good, I tend to use it more than my digital camera these days and I am taking more photos now than I used to, partly due to the iPhone.

Sound
The sound quality is what you would expect from a mobile, but is a big step up from the 3GS, I prefer to put it on my docking station to play music rather than play it out load but I find the quality of sound when using earphones very good, with a crisp clear sound. I travel about a lot so use it to listen to music on the move and find its much much better than a normal mp3 player.

Battery
Of course the only minor downfall is battery life, a lot of iPhone users complain they want a better battery but so far Apple have not really done it. I tend to charge my phone about once a day, which is kind of annoying but to be honest now I don't even notice it, especially since there are plug sockets everywhere these days even on trains & coaches to charge phones and the phone comes with a plug adapter aswell as the USB which is very useful. I have also bought an emergancy charger which runs off normal AA batteries so if I am out & about and it dies I just pop in the emergancy charger which is small enough to fit in my pocket or bag this gives me an extra hour & 30mins approx and it only cost me a few quid.

Apps
I love the fact there are so many apps available through iTunes, you really can find everything you could need, that's one of the reasons I stuck to the iPhone when I upgraded, they are literally thousands available, a lot of them free, and the apps make life so much easier as I can do pretty much anything whilst I am out & about. Apps are very easy to install on your phone, once you've downloaded them to iTunes its just plug in my iPhone with the USB provided and it does it automatically, I can also download apps on my phone and they will automatically go on iTunes which is useful too.

Overall
I think I have covered the main bits, overall I highly recommend the iPhone 4s it is a perfect phone and I have not had any faults with it. I will upgrade to the 5 when its time to but for now I am more than content with my 4s and I always tell everyone of my friends to get one especially when they are moaning about their blackberry not working.