I have a 3G iPad and it's great being able to get your email etc when you're 'out and about'. The UK, being a physically small country and for historical reasons, has pretty good mobile coverage so even in 'remote' areas of the country, there is good coverage. I live in the North East of Scotland and I get good 3G signal strength. In the UK, too, many operators offer an 'all you can eat' package for mobile broadband (i.e. no monthly limit). That's the good news.
The bad news is that, for whatever reason, Apple have limited in-app downloads to 20 MB over 3G. So this means you cannot download movies, large apps, large books, large music files, large audio books. This is a real pain and, to my mind, seriously undermines the usefulness of having 3G. The one exception seems to be the Internet radio app TuneIn which will stream continuously over 3G - not sure how they get round this problem (my iPad is not jail broken, I should add) but it's great that they do.
Hopefully Apple will remove this ridiculous restriction - it's like selling you a top-of-the-range sports car and then telling you that you're not allowed to drive it on the freeway....
In terms of the storage capability, I think this will depend on what you plan to use your iPad for, of course. If you want to download a lot of movies (not over 3G ;-) or music (OK - you could download individual tracks over 3G) then you're clearly going to need a lot of storage space. A movie can take about 1 GB or more (if you want the high definition version) so clearly a 16 GB iPad will quickly fill up.
If you're using the iPad for document preparation, web surfing, emails etc then 16GB would probably be fine.
However, whatever the restrictions, I don't think many of us have regretted purchasing the iPad. As one of the early reviewers said, "If you don't want an iPad, then you haven't tried one". I hesitated before purchasing mine - hey, you could buy a good laptop for the money - but, after only a few hours I knew I'd made the right decision.
If you're going to use it for writing a lot of documents, emails - posts to the iPad Forum !! - then a 'must-have' accessory is a Bluetooth keyboard - preferably a 'folding' one that you'll carry around with you all the time. The on-screen keyboard is OK for small posts but, certainly if you're a touch-typist, something 'physical' is really needed.
Hope you enjoy your new iPad - I'd be interested, if you get the iPad2, to hear your impressions - I can already feel my credit card vibrating in my pocket.....
Tim
Scotland