I'm still using an old phone with Three's mobile internet access bolt on.
It is an LG U890, it came with a usb cable and acted as a modem from day 1 (I didn't get much use from it as the costs were so high).
Newer handsets can take advantage of the increased data rates in some areas, but it is important to check that the modem facility is there if ordering an upgrade or starting from scratch.
Using the older U890, I get download speeds (according to the broadband speed check sites) between ISDN and Satellite (400k). Fine for general browsing and email, even some downloads (although the 1Gb limit means that this is not going to be the main activity).
As I understand it, on the three network, if a phone used to allow modem access on the older service, then it should work at the older (ISDN/Satellite) speeds using the new bolt on.
This is all great, but my new laptop came with Vista and didn't talk to the old phone as a modem! If you have this problem see the Vista/Linux page for a work around to allow both of these operating systems to access the internet using an old phone or un-supported modem.