In all honesty, I've not found a huge amount of difference since installing it. Although 100 touted features are no doubt lurking beneath the screen, waiting to pounce on my when I'm next writing a text message, I've only really noticed a few. I like the geo-tagging on my photos; I can now view them by where I was when taking my pictures. I also really like grouping my app icons together into clusters; News, Games, Social, etc. However, the ability to change the background to your own image as opposed to default black actually irritated me. I hate the agony of choice that surrounds me everyday - I don't know what to choose! 1984 didn't sound to bad to me and I wish Henry Ford hadn't bothered offering his Model T in any other colours.
I'm also not fussed with the multi-tasking, which has been a long-standing complaint amongst iPhone users. It's not a computer, it's a phone. There cannot be THAT many things which you need to be doing simultaneously. OK, it's good that I can leave Skype on whilst doing something else. But what bothers me is that once you've opened an app, it doesn't close when you hit the home button. This means that you can inadvertently have a whole host of apps running at any one time, draining the already fickle battery life. To make sure they're really closed, you need to open what is effectively a 'toolbar' (ironically a key feature of any Windows OS) and close them down by holding them. FAFF.
This irritation would be eradicated if Apple pinched a few of the developers at Mozilla; shouldn't be hard, they work for free. This Tab Candy feature is a really intuitive way to organise your tabs into specific tasks when you're working. Saying that, the iPhone is a phone. Which should really be best at making calls.
Try the alpha version here.