Got my iPhone 3GS 32GB on release day. I got an iPod Touch a few months before that so that I could try out the OS/GUI and see if I would be happy with it before switching from my low-cost Sprint SERO plan to AT&T. I'm sure I'm forgetting something, and I can't remember the exact order of some of these, but here goes...
Treo 700p (Sprint)
Treo 650 (Sprint)
Treo 600 (Sprint)
Treo 300 (Sprint)
Samsung SPH-i300 (Sprint)
Kyocera 6035 (Verizon) - returned it within 30 days (color standalone Palm OS PDAs were out, so it was too hard to switch back to B&W)
HTC Touch (Sprint) w/Google Android build (not bad)
HTC Touch (Sprint)
HTC Mogul (Sprint) - returned it within 30 days
HTC 6700 (Sprint) - returned it within 30 days
Nokia N-Gage (T-Mobile pay-as-you-go)
Sony-Ericsson T608 (Sprint) - hoped to get by with tethering via Bluetooth to my Tapwave Zodiac
Motorola StarTac (Verizon)
Most of my experience has been with Palm OS. Despite trying several times, I was never happy with Windows Mobile. I liked the idea of the Danger hiptop/Sidekick, but T-Mobile service wasn't good in my area, and I didn't like the closed app situation with them, so I never tried that out. I also never tried the Blackberry.
I'll post more details about my thoughts about the Palm OS, webOS, and iPhone OS/GUI in a separate thread, but I'll just say briefly that there's a lot of usability/efficiency things I like about the classic Palm OS, but webOS is a whole new thing by a different team that wasn't really a part of the "zen of Palm" in the past. Hawkins is gone now and the new team appears to be a bunch of ex-Apple people who created the Palm pre as an iPhone-clone that tried to one-up the iPhone. It succeeds in some ways, falls short in several others, and doesn't respect the good qualities of the old Palm OS, so I look at the Palm of today as a whole new company, for better and worse.
Perhaps the biggest reason for me going with the iPhone is the wealth of apps available. It's a modern OS/GUI with a ton of apps. If Palm and Google are successful (and I hope they both are), there should be some great choices available in the next year.