Disclaimer I work for a mobile phone company so have unlimited 4G, but here’s my own experience with it to date and and hopefully a non biased view on the UK 4G market.
I’m mobile/homeworker and the nearest cell site to my home was upgraded to 4G about six months ago, since then I’ve used my mobile phone for tethering quite a lot and its more than good enough to support audio conf/video conf on MS Lync. Also for some customer presentations from remote webex speakers/presenters where it’s been excellent.
Whilst you have those big headline upload and download speeds the largest improvement is the fact that 4G is a data network from the outset rather than data tacked on top of a voice network, which means a huge reduction in latency so you will see a general reduced response time, things just feel more nippy.
What still has to be done is sorting out voice at the moment if you make or receive a call the phone will drop back to 3G to make the call thereby screwing up your wonderful 4G connection. It won’t be that long until there is some sort of rough consensus and VoLTE or similar comes out which sort that issue out and most likely improve battery life a bit too.
As for the question to replace broadband my own personal view is if you look at anything new to market it goes thru a number of phases, early adopter, mid adopter and so on, each with their own price points, so I’d expect that during the mature phase of 4G that there will start to be packages appearing with some hefty download limits built in.