Engadget compared the popular mobile platforms with one another, including the ones that haven't yet been released. They took a look at Android Cupcake, BlackBerry OS 4.7, iPhone OS 3.0, S60 5th Edition, Palm webOS, and Windows Mobile 6.5.
In terms of the user interfaces, Engadget concludes that the iPhone is the one with the least problems, mostly because it had a fresh start and wasn't burdened down with years of non-touch specific user interfaces. Engadget says that while Windows Mobile and the BlackBerry OS made improvements in that area, they still have some work to do compared to the iPhone (let's just leave S60 out of this paragraph to save it from embarrassment). The Palm Pre took a long hard look at the iPhone, and added lots of ideas of its own, but obviously it hasn't been tested yet.
In the core functionality department, the iPhone's competitors do a better job, according to Engadget, mostly because they can all multitask. In addition, while Android and webOS slide an unobtrusive "tray" into view when a notification is received, the iPhone still stacks modal upon modal dialog whenever a notification is received. The word "archaic" comes to mind.
When it comes to third party application development, Apple really shines, Engadget writes. They made it very easy for developers to write, publish, and earn money off their applications, something the other platforms are only starting to do today.
In the end, a very important factor in choosing your mobile platform is personal preference. For instance, I have a deeply rooted hate towards touch screens (the iPhone's included), so I most likely won't buy a device without a physical keyboard for a while. Many of you will have these very specific pet hates and loves that influence your decision.
Still, Engadget's overview is pretty detailed and handy.