I'm no expert on these matters but I recently bought an O2 mobile broadband dongle for a recent holiday.
It was 3Gb (I think) for £15. I did a thread on here asking questions about it and found out that for my 3GB, I should be able to browse the web for approx 2 hrs a day for a month.
When I used it I was surprised tbh as I expected it to be crap, it wasn't.
It was much faster than dial up and marginally slower than my home connection. (even drving along in the car)
I watched plenty of vids with it (redtube) anf generally used my laptop as per normal.
If you're gonna start downloading and stuff then (from what I'm told) it won't last long and you'll need to top it up again.
I was very impressed