I borrowed the 'Three' dongle mentioned in the article from a train journey from Glasgow to Leeds.
Coverage was apalling, the one benefit you will get from the other networks is a 2G GPRS signal where a 3G one is not available.
Three used to offer 2G GPRS if 3G was unavailable, but this is no longer in place, so if there's no 3G signal, you've got no connection. I would say that in for the total ~4 hour journey I was connected for about 15 minutes in total. I took a different route on the way back, and still had no real joy,
If you're in business, and travel about, and require a dongle, I'd avoid the three one. I'd rather have a 'normal' network with some connectivity, even if it is just 2.5G GPRS than relying on finding a 3G connection all the time through three.