Symbian, in its day, was a decent OS. Still is, to be honest.
It fell over because it was a general (ish) purpose OS, and never really had a front end/UI. That kept a lot of manaufacturers away from making consumer devices for it, leaving pretty much only Nokia. Nokia had, Sony style, been resting on its laurels for so long, it took its eye off the ball, and wasted too much time and effort coming up with idealistic replacements, where as what it needed was a better UI on Symbian. Then the in-fighting started, then it all went wrong.
The likes of the N95 was well behind the competition for smartphones upon its launch, and only sold well because, again Sony style, some would only consider Nokia. Gay. The N95 wasn't a bad phone, most of what it cleaned to do, it did OK, some it did well. But it was clunky to use, non intuitive to non Symbian users, and was a bit buggy early on. Around the same time, I used a Windows 2003 phone, and it was a breath of fresh air in comparison (but not without its own fundamental flaws). Windows improved, Symbian stagnated, then along came Apple, which was a rule changer.
Where are we now?
Symbian is a has-been
HP/Palm came and went, nobody wants webOS
Gooseberry looks to be a thing of the past, rightly so, they haven't kept pace, and now their prime market is dumping them fast
MS are toying with dumping WinCE (good!!) and moving to an NT based kernel, probably the Windows 8 one
Apple are still probably leading innovation around user interface, and you have to admire the stability
Google are selling more than all the others put together, much to MS's
delight $, but seemingly always playing catch-up to imitate Apple
What do I use now? I've been using my current iPhone for about 2yrs, no plans to change it. Ironically, it still talks to Microsoft's email system far better than Windows phones I've tried
What would I buy if I broke mine tomorrow? Hmmm, not much choice for my needs. Android doesn't have a good email client for anything, and email is critical to me, so would probably currently be another gayPhone. Which saddens me.