I've had the 3 mobile dongle for 18 months now, it's been a frustrating roller coaster regarding speeds obtained, but, it would seem speeds are getting very better as time progresses, (except in one particular postcode area I might add!!).
Three have recently been upgrading my 'home' location, checking, my recent speed tests are now showing regular 1.5mb downloading speeds with an average, in normal use, I'd say of 0.7mb and being quite acceptable for my needs.
(yesterday while testing, the system registered over 7mb!!...is this a promise of things to come??)
Abroad I use a local Simm and EGPRS which is the compressed version Nigel speaks of I think, I find this fine for simple tasks such as email, banking and dare I say forum surfing!