The HTC Desire has a couple of differences to the Nexus One, but is essentially the same hardware except instead of a trackball it has a trackpad type of thing.
Most of the difference is in some HTC tweaks to the OS. It runs the HTC Sense UI, does USB tethering out of the box.
It's a big phone, but most Smartphones are, it's pretty much the same size as an iPhone 3gs (iPhone being 4.5 x 2.4 x 0.48 inches and a HTC desire being 4.7 x 2.36 x 0.47 [both weigh135 grams]).
I've been running my Nexus One since the begining of Feburary and can definatly say it's a fantastic device.
I have a couple of niggles with it, but they are all software based so can be sorted. (the perils of fearly adoption :dead
1) No way to configure a proxy server. [This is an android shortcomming and is a known issue]
2) Will not send MMS messages if the photo is a landscape one [weird, but I think this is more to do with me setting up my APN wrong]
3) Doesn't support tethering via USB or bluetooth [this is a nexus one shortcomming for some reason. Android supports it]
As I bought my Nexus One direct from google without a contract, and my Orange contract is up in a month or two, I am actually seriously considering upgrading my Orange contract to a HTC Desire [which would be covered by Orange Care] and then rooting that to mess around with [don't want to root my Nexus at the moment until I have a spare phone or a method of getting a replacement should I kill it
]
Battery life is pretty good, as long as you aren't caning the WiFi & GPS. In a normal day I will usually have 75% battery left when I put it on charge when I go to bed. That's with WiFi on in the evenings at home [turned off rest of day], GPS off, bluetooth on, syncing every 10 mins with my e-mail accounts, and used as my MP3 player in the car via bluetooth (so about an hours worth of MP3 play at max volume output via bluetooth speaker).
Having said that if you have WiFi turned on all day along with GPS and are messing about with it all the time (like I was in my first week or two) you'll find yourself draining the battery inside of a day.
Has a single micro-usb connector for charging & data, aand will happily charge off any USB based charger or PC with a normal USB micro cable (I actually use the one from my old Nokia E71).
The headphone socket is a standard 3.5mm jack, so you can just plug any old headphones into it instead of having to deal with adaptors or the handsfree kit.