I've been using 3's Mobile Broadband extensively since Saturday. As a trialist I can use it as much as I want, it's truly unlimited! In the real world, 3 offer packages of 1GB, 3GB and 7GB (Lite, Plus and Max respectively) - unlike their handset-only offerings such as X-Series, there isn't the option of unlimited ("fair use" or otherwise).
I've taken advantage of the unlimited-ness to download a whole bunch of videos using Miro. So much so, that I'd have easily used up all of the 'Plus' allowance in one day! Not something I'd recommend paying customers do.
The fact that I can so easily download that much over the mobile network in a matter of hours is kind of amazing considering what speeds were like just a couple of years ago. Even though I can't get the the 2.8 Mbps speed that the service goes up to in Turbo coverage areas, I'm still getting speeds comparable to what a BT landline offers in this area. And as this neighbourhood doesn't have cable, it's a welcome choice over ADSL.
As I had Miro downloading videos constantly and consistantly for a couple of hours, I was able to see where best to site the laptop and sit, myself. The garden turned out to be the best place - but it didn't improve speeds as much as you'd imagine. Taking the USB modem out of the port, Blu-Tacking ('Blu-Hack') it to the top of the laptops screen and connecting it back up via the supplied USB extension cable - is by far the easiest and most effective way to boost your broadband. This worked better than the 'garden' method on it's own. Even the 'upstairs room facing 3's nearest mast' method (which incidently is very, very near*) isn't good enough on it's own.
*Thanks to OFCOM's Sitefinder (it's useful but rubbishly implemented!)