I run an 3G, 16GB iPad 2 for work, and a 32gb iphone 4.
The ipad has a PAYG data card from THREE mobile, I spend most of my time working in central London and went for this, as it is reputed to have the best access in London. That said, on holiday in Cornwall this year, it had better reception than the other ipads in our group on vodafone. I'm really pleased with the performance, as Peter says if you want to stream the BBC, forget it, but for all other things its great.
I go for the £10 a month for a gb of data, so far, and I've had it 3 months, I haven't used above the data limit in each month.
In terms of data storage, I have all my work files synced via dropbox, which is limited to 1gb in the dropbox settings, I've also got Pages, Number, and keynote which are fantastic for meetings. I don't think I'll get anywhere near the 16gb limit of the ipad.
just my thoughts - but go for it, you'll wonder how you existed without after a while