I got my phone a couple of months ago on Orange. I initially had Dolphin with 500MB 'Inclusive Internet' - anything above that and you start getting charged.
I'm now on Panther 30 with 'Unlimited Internet' - FUP 750MB anything above and they can start to restrict your speed, but crucially (for me) no extra charges).
I have wifi at home and work and last month I used 60MB via Orange and 500MB(ish) via wifi. My usage is pretty typical I would guess: some browsing every day, occasional 10 mins streaming, fair few apps downloaded and not much syncing in the background.
Roughly, a web page takes 1-1.5MB, 5 mins streaming takes 10MB so it can soon mount up if you don't have wifi.
I'm much happier now I have the 750MB FUP rather than the 500MB hard cap with charges but with having wifi at home and work my actual Orange network data usage is slight.
I'm testing how much I use this month by not using my work wifi - so far I'm upto 45MB in 3 days so I think I
will get near the 750MB this month
Just to add, some very conflicting information comes out of Orange regarding the FUPs and any charges if you have the 500mb 'Inclusive' Mobile internet. I personally only trust the terms and conditions :
See Section 5
http://www1.orange.co.uk/service_plans/paymonthly/termsMarch2010.html