I tried Symbian (Nokia N95) looking for an easy and open OS, but was disappointed by the messy and un-intuitive way things were done (seems like everything was done "top-down" ie: instead of listening to what users/developers want, Symbian/Nokia called the shots, and eventually confused and p*ssed people off).
Then I tried BlackBerry (Tour 9630 on GSM), and was impressed by the solid, intuitive OS, but in the end the BIS data restrictions killed me ie: I was trying to use it with only a APN (TCP/IP connection) and kept having problems. Plus RIM didn't have any sideways sliders, like the Droid, so lack of hardware choices was limiting as well..
Now I've got a Moto Milestone, and so far I'm really impressed by the simple/easy way things are done with Android. It looks like Google works it "bottom up" ie: they listen to the users/developers and base the OS on their input. There's still some kinks to work out (ex: running the same OS on a variety of hardware mfgs can sometimes run into problems for devs, and result in unstable apps) but Android is in it's "Windos 95" stage right now, I'm sure give it a year or two and things will be running much smoother...