I use the Tooway service through Avonline (as they were the cheapest). I have the unlimited version and use it for work to send large video files to clients to approve - and my wife uses the download speed for iPlayer etc. You will find general web browsing a little frustrating because of the very high latency with the data signal - probably similar to the delay you might get with a 512kbps - 1Mbps connection.
I'm actually working at a customers site where they have Tooway right now (indeed this message is sent using said connection - near Honiton). I don't think the browsing latency is a big deal at all - it's a brief wait, but pages then arrive very quickly. I think it's surprisingly good for a service with 800ms latency.
I've streamed iPlayer etc - a little less consistent but it works.
I'm actually quite impressed with the tooway service.