I use a 3 dongle now and again. Good coverage, works well, and not stupidly expensive! 
I find 3's coverage utterly useless, so I don't really use them. Like Vodafone, seems to work well in urban areas, and near motorways, but generally non existent when rural.
I now tend to stick to O2 based carriers, as coverage is usually very good, although not as much 3G as some of the others.
Pretty much tried all of them over the years, with HSDPA speeds in mind for me, Vodafone seem to fit the bill for me, pretty much gave the best for consistency in terms of through put vs Signal Strength, O2 again good but couldn't live with their charges if you hit the monthly limit, and 3 was a bit of all or nothing.
Currently using T-Mobile PAYG on a £20 for 6 months deal for unlimited with no additional charges, that works out at 11p a day, great value, meaning I can leave the sim where it belongs, Data use only as when I need it, plus the added bonus of using 2 Networks at HSDPA T-Mobile and Orange, fairly consistent in speed terms gives around 1.2 to 2.4 Mbs, with around 3.6 max, very rarely reduces down to 3g or GSM.