Unlocked and on pay as you go with T-Mobile, but reading up on it Vodafone seem to be real good these days for PAYG.
Unlocking the phone is easy and free. Rooting the phone is easy and free. Installing custom ROMs is easy and free.
Sometimes the build feels cheap. (Volume & power buttons are a bit plasticy.)
I've had a couple of friends with dodgy ones that needed replacing. (Not good if it were to break after you'd put on a custom ROM.)
Camera is poor quality in low light.
THE most annoying thing is that the WIFI doesn't automatically reconnect after the phone has been in sleep mode. It's just a matter of installing an app called "blade wifi fix," but that's just not as smooth as it should be. It takes about 15 seconds to reconnect to my wifi after sleep. Not good for someone impatient. 3G is instantaneous though.
The processor is "only" 600MHz so it's not as quick as other nice phones. Most things run smoothly but some stuff struggles. (E.g. Angry Birds is playable but irritating as there is significant slowdown in places.)
Facebook, twitter, browser are all smooth.
Flash is not supported, even on Froyo.
/flowofthought