I used various models from Blackberry (from the pager model and the 900 series all the way up to the Pearl) for oh...about the last 5 years or so. I was on a sprint 6700 WMD a couple of years ago, when RIM was involved in the lawsuit and I had to see if WM was a viable alternative for my company. I've run Treos, both during and after it was Handspring.
Here's my analysis:
Windows Mobile:
The Windows Mobile phone could do amazing things. It could do obscure, rarely used, and even more rarely NEEDED things amazingly. If I wanted to create a VPN into my network and get a 3rd party command line parser/forwarder tool so I could manage my servers from my phone, I could do it. It was great at doing the things I never really needed to do. And it sucked at making phone calls, it sucked at syncing, and it sucked at email. What did I REALLY need it to do? Make phone calls, and get email. All the cool features in the world don't count for anything if I never use them and the device ***** at what I DO need it for.
Treos:
Much the same as above, both the Windows and Palm OS versions. Cool features, rarely used. Decent email support, but crappy email interface. Phone was only so-so. Screen sucked hard. Good 3rd party apps that I rarely used, cause if I need to create content that bad, I've got a friggin laptop.
Blackberry:
Unparalleled email device. Blackberry Enterprise Server was more reliable and functional than Goodlink/Exchange ever was. Kill, lock, provision, policy mgmt of the phone, lots of good stuff there. Keyboard was great (the full qwerty one). Sucked for making calls, sucked to look at, media playback was horrific. The Pearl? aesthetically was pretty nice, good form factor, small screen, keyboard not as effective, media playback still kinda sucked, but not as badly as the full models. I liked my Pearls (had one with Tmobile, one with AT&T) a lot for what they were, but I never wanted it back after getting my iPhone. Not to mention the Pearl's dataplan wasn't exactly cheap.
iPhone:
I finally have a device that does most of what I want, and does it well. Great for email, great for media, great for web, phone is about as good as I've had in a mobile device (last phone i recall getting better reception out of was an old Motorola v60). I can view Word, Excel, PDF files with ease. I have a fantasically complicated Excel workbook that has string handlers and calculations I thought for sure would cause the iPhone fits, but nope, it works flawlessly. Is it perfect? nope. I want task lists. I want better calendar integration. Is it better than the rest for me? Yes. I don't need to have an overcomplicated device that CAN do things I don't need it to. I want something so I don't have to carry a bunch of crap around.
I hardly ever tethered my other devices, so as a bonus, I got to go from a data plan that was $80/month to one that was $20. Doesn't take long to pay for itself at that pace, so I'm not worried about price tag differences.
The simple fact is that Apple made a device that does what I need it to do well, well. Everyone else seems to be trying to sell their device based on the merits of a feature set that I rarely had need for while providing a subpar set of core features that I really want to work nicely.