I've used them both on work stuff.
I have a desire, because I write applications for my phone. If I had an iPhone, I'd need to buy a mac, then pay apple money, before I could even run applications I wrote myself on my own phone. Then if I wanted anyone else to be able to run them, I'd have to ask Apple nicely, and wait a couple of weeks before they could.
With the Android phones, I download the free development kit on any computer, write a program, then I can put it on my phone, or give it to other people to play with straight away.
Android is a lot more open in loads of other ways too – if you want to put music on it, you can use pretty much any piece of software, you're not locked to itunes, if you want to run a different web browser, a different sms application etc, you can choose to, whereas on Apple, you can only do what Apple let you do.
It'll be interesting to see how things pan out in the long term – in the usa, Android phones outsell iPhone currently, so potentially may end up with more applications also (especially given it is so much more developer friendly).
On practical stuff, the two are roughly comparable – both have only okay battery life, which is a pain. The gps in the desire is probably a bit better (it is very very good, the last iPhone I used a gps on was pretty inaccurate, that was only a 3g not a 3gs mind).
The android phones have widgets, where things like calendar, weather, Facebook updates, emails etc can be shown on your home screen, which apple don't let you do. They are currently a lot better for gps apps on the bike because iPhone doesn't support multitasking yet, meaning that if you get a phone call, the gps drops out.
From a usability point of view, both are pretty similar – decent web browsers, phone, email etc all easy to use. Much of a muchness really now.