My Mother's bloke has a Desire, I love it, but after a lot of research I've decided to go for a Samsung Galaxy S over it. Similar price and several better features, plus it looks hotter IMO.
Not an iPhone fan at all, I just prefer the Android OS to the iPhone one.
I'd try to steer clear of Windows Mobile powered phones as well, I don't know anyone who is happy they went for it over Android or an iPhone.
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Bit more info on the Galaxy S, as that's the only one I really understand. Here's the one I've been looking at:
No down payment, £30 a month, 600 minutes, unlimited texts, 500mb of data.
On the Galaxy S, I believe you can set up a number of wireless networks, so if it comes in range of any of these networks, it will connect to that and use it, so you don't have to use up any of the 500mb data limit.
Also, I'm told it's best to buy from either mobiles.co.uk or the carphonewarehouse, because you can get some really good deals on them. (8 1/2 months half price on the example above at mobiles.co.uk).
However, mobiles.co.uk have had some shocking reviews....