The iPhones 5S is a great device, with an extreme slim design and superb build quality. I'd definitely recommend it to any current iOS user.
However, moving from Android; you may feel as if it's a downgrade. The small screen size will be the first thing you'll notice, at 4". Next thing, is the limits you have when it come to customisations. No widgets or anything on your "home" screen/springboard; all you can do is organise app icons. You can't even download/upload files from the Safari web browser (zip files/mp3/docs), bluetooth to any device; or even download torrents. To transfer music to your phone, you need to connect to your computer and sync. You also can't tether 3G/4G from your iPhone, unless your carrier directly supports it via Apple. Another thing to note for some people, no installing from "Unknown Sources", if you know what I mean.
Another thing on my mind is that you don't have true multitasking; I was on the bus (long daily journey), loading a web site via 3G, it was going slow so I decided to do something else whilst it loaded. I opened up my email app, then I noticed that the browser stopped loading content. I went back to the browser, and the page completely stopped loading, and left a blank/white page. The only app that can multitask is the music app, alongside few system apps.
All the above limitations of an iPhone can be lifted if you jailbreak your phone. I have an iPhone 4S as my primary device (alongside all my other Android devices), and it is jailbroken. It can now do pretty much anything Android can; without a jailbreak - I would have sold my device after a month or two after owning it. Apple completely restricts you.
There are more limitations to iOS, but the new iOS 7 addresses a few things (including multitasking, but not true multitasking). However on iOS 7 there is currently no jailbreak available yet; hence why I have not updated my version of iOS; I'm still on 6.1.2 (last untethered compatible jailbreak version).
Before you get an iPhone 5S, I'd weigh the benefits and drawbacks. You could also wait for the Samsung Galaxy S5, or even the iPhone 6 next year. iPhone 6 should have all the features of the iPhone 5S, and it's software (and fingerprint sensor) would have matured by then. I'm sure Apple will bump the screen size up again from 4", since they can't afford to stick to 4".
It's completely your choice, but that's my advise/suggestion.