Probably everyone has used, or at least seen a Symbian based phone.
If you're a normal user and don't know what i mean by "Bluetooth Stack", please stop reading or you just might not understand what i am talking about and get angry at me for no real reason.
For myself, i've used Symbian based phones for ages. The first time i realized that i have a Symbian phone was when i got my Sony Ericsson P990i.
This mobile device sucked so hard, i got nosebleeds because i got agitated that much. Once i reached about 100 short-messages (SMS), the whole Phone got unresponsive without even having the SMS-Applicaiton open.
That was almost 5 years ago.
A lot has changed since
but nothing good for Symbian happend. Why do i say that? It's easy to find out yourself. Go into a store that has one or the other and compare the usability.
Symbian still follows the design-concepts from the past decade, while Android and iPhone both have their pros and cons, but none of them has as many cons as Symbian. While using a Symbian device, i constantly have the feeling that something is somehow missing.
So what mobile device makes me comfortable using? Nowadays we got mobile operating systems that rule by far over Symbian: Android and iPhoneOS/iOS! And yes, they're both worthy (dominating) opponents compared to Symbian.
Both are better to develop for and both are more user friendly. J2ME vs Android SDK or iPhone SDK, i don't really have to explain anything, do i?
What's the difference between iOS and Android?
Google Android is an open environment where you have a variety of mobile devices/smartphones which can run on. I personally ported a pretty early alpha of Android to the HTC BlueAngel. Nothing fancy happened but i got it booting!
iOS on the other hand is just for Apple Devices. No third party vendor is allowed (yet) to use iOS on their devices. But that's just Apple's concept. They write operating systems that run (mostly) seamlessly with the devices they produce (and only them). Those are the ingredients and a lot of people love the soup.
What about the development?
First we got Xcode which encourages People who already code for OSX through its Apple-ish structure. The problem is, you can only code iPhone Software on a x86 Mac, but if you've worked with Objective-C and Xcode before, developing iPhone Apps mainly differs in layouting and memory management (Garbage Collection). Some people might say that it's bad to manage the memory of your device all by yourself without having a garbage collector, but in my humble opinion, it's just not that bad! Finding memory leaks is a piece of cake and therefor no problem at all. Sure, if you come from a Java background, you might have your typical complains ;)
Developing for Android on the other Hand is alot sweeter. The only problem is, that you really notice the garbage collection in performance on most devices. Using Eclipse, NetBeans or IntelliJ for development is nice, at least one philosophy that Apple could have adopted, since no one really needs the Interface Builder and the hard-boiled environment. If you come from a Java-background, it's just a matter of minutes to get your first App running in either of the named IDEs.
Well, it really depends on your taste what to favor over the other, but for non-developers/non-hackers i'd recommend an iPhone while techies who need their freedom would be better of with an Android based phone.
And if "why will Symbian go down?" is still your question, you didn't understand anything i just wrote.