Mobile broadband - I'm confused !

[#1]

Can anyone tell me what Mobile Broadband really is ?  I get the impression that Mobile Broadband is not limited to mobile phones, and that it can be used to connect a laptop to broadband when roaming.  If that is correct, does one really get broadband speeds with such services and if so, what sort of speeds in reality and how good is the coverage ?  Do you actually need to find a hot-spot, or is the claim from people like 3 mobile that it is available for 80% of the country  just marketing hype ?  Is it practical to use this sort of connection instead of a land-line based broadband service  or is it really intended only as a back-up ?
If any of the above questions are positive, does Plusnet have any plans for offering suchg a service ?

[#2]

I have used the mobile broadband in a few cases, one by using a Linksys 3g router allows a (Vodafone) data card to be used as the connection and shared by those who connect to the router - either wired or wireless connections.  (The Linksys was used on the Gadget Show last year when they did a feature on accessing the internet when outside)
Other standard ADSL-Modem routers - Draytek make one I think - have a secondary USB connector and can kick the 3g device into use when the ADSL goes down.
I've mostly used these within the M25 area, so there would be more chance of getting a 3g signal.
Also recently the 3 network and T-Mobile announced they would share their 3g masts - so there is more chance of getting a decent signal on either of their networks.