Today I updated to IOS 7 on my iPad (I use a Samsung mobile). I'm not actually sure if previous versions have as many accessibility features, but I found the most useful feature for my vision was the "Larger type" text feature. It gives you the option to increase the font size in any app that has enabled the feature. Of course I made it almost to the largest and then I tested out all my favourite apps.
The standard email app is now awesome. The font is enlarged in the content and title of the email and it makes it really easy to use and puts less strain on my eyes.
The other app I barely used previously was the standard calendar. I found I could never read at a glance what appointments I had so I never used it. The design has changed and it is visually appealing plus the appointments are larger so I can actually see what I am supposed to be doing!
Other apps I use that I've found to be easier to see are: Music, eBay, Maps and Podcasts. I'm assuming the update would also be useful for iPhones, but I found the biggest problem I had with the iPhone was the size of the screen in comparison to the text sizes, which is why I switched to the Samsung.
IOS 7 is definitely worth an update.