I have it with t-mobile. (WebNwalk+)
Providing the cell your in isn't to busy and both the cell and the device you are using to connect supports HSDPA you can expect around 512Kps actual sustained throughput although it Peaks higher.
I've had problems with MSN messenger signing in on t-mobiles service though. Not sure why, they insist they are NOT blocking it, and some clients seem to connect better than others. (the official mac client sometimes will when adium and amsn don't)
T-mobile (as i think do the others) use a compression Proxy that reduces image quality in order to speed up loading as well as use less data but you can then get the full quality image by refreshing the page (or theres a key combination to do it) I think most of the networks let you disable it though.
Depends what you want to use it for, as a main connection I wouldn't recommend it but its fine for surfing the odd small download.
if your in HSDPA coverage the ping is low enough to play world of warcraft
(often in the 150ms-300ms range) if you really wanted to... I have done a couple of times when I was bored if the voice chat is off it uses hardly any data either litreally looking at a few MB per hour
