I’ve just signed up to 02 as I live in a shared house with about 5 other people and getting a proper connection would be too much hassle.
First off, speed is very variable. It depends on what networks are around for it to connect to. If it can connect to the 3G network, speeds aren’t too bad (10k-20k/s). If it can only connect to the 2G network, speeds are abysmal. It does also drop the connection sometimes.
It does an annoying thing called “image optimisation” where the quality of all jpegs is reduced to use less bandwidth. You can up the quality of individual images (say, if you’d copying an image off the ‘net) but it’s a faff and it doesn’t always work. You can’t switch it off either unless you’re prepared to do some tinkering with your browser’s configuration. I’m still trying to figure out how to do this with Firefox – the previous fix people were using (an add-on that lets you reconfigure the header info for web pages) isn’t compatible with the new version.
If you have a CRT monitor then the dongle can interfere with this in the same way as a mobile phone.
For what it is, it’s a bit expensive and unreliable, but it does seem to work OK for me at the moment.