I have the 3 12GB PAYG dongle, coverage seems to be pretty good (central Scotland) and download speed are ok, (feels more like 512k broadband than 3Mbps) the only issue I have are frequent disconnections even in strong signal areas, sometimes connection will last 5 mins, sometimes over 1hr in the same area, still its way better than 56k that was the norm for home connections only a few years ago.
I was able to stream a video (not buffered) for over 20 miles without disconnect.