I'm currently with O2 on a Simplicity 20 for iPhone plan. It's a rolling contract which I've been on for what, maybe three years now?
It's £21.07 a month for 600 minutes, 1200 texts and unlimited data, a plan which no longer exists anymore.
For just under an extra £4 a month I could have the new 4G Simplicity plan which includes unlimited calls and texts, but the data drops to 1GB.
I'm undecided at the moment as I believe 4G will only be of any use in a city like London. When I got outside of London there are still many places such as where my folks live that I can barely even get Edge, let alone 3G, so in many aspects it seems pointless.
With unlimited data being quite hard to come by these days, at least for cheap, it seems a shame to give that up. But that said, I probably end up using WiFi connections enough that I doubt I'd ever really drift into using 1GB of data anyway.
I also feel that 4G will probably get competitive between the networks as I think most people won't see the benefits to want to switch. However they may do this more with the phones they subsidise on contracts.