Based only on light use, it works well enough, but not quite as good as a normal satnav, mainly because the screen is a bit small. Also it is not quite as fully-featured as other satnavs; for example, it will recalculate routes and warn when you exceed the speed limit, but has no speed camera database (not that I have found anyway).
It does not use broadband because it has GPS built in and that functions well.
I have no idea what the Vodafone version does that the free one doesn't. Try the free one and see - I can think of no reason why it would void the contract.