I wouldnt have done in the past, but now i would with my iPhone etc and i think many others would aswell.
I have an iPhone 3GS on Orange and they give me 250MB a month as im PAYG, and when im working etc i can use that up quite fast and i have done in the past, then once i go over that allowance, they then take it out of my credit without me knowing its ran out.
I'm getting an iPhone 4 at Xmas but this time probably on T-Mobile as they will give me 500mb internet a month which would be plenty for me, but even if you go over the 500mb on T-Mobile, they wont take it from your credit or charge you, just monitor your usage and restrict you if blatantly abusing the service.
So IMO, Yes data bundles and size is important to me.