My parents live in the States and I got my dad to bring one back for me last weekend, so have been using my iPad in the UK for 4 days now. Quite timely as am away on business and have had ample opportunity to put this to the test as a replacement for a laptop when travelling.
I'm using the 3 MiFi, and I'm really happy with it. It's very small and light, charge lasts all day easily even with heavy usage and the internet seems to fly - and my hotel doesn't have wifi and not a great place for reception. On the PAYG plan which I think will suit well - although 3 will unlock it for you.
One thing I have discovered - so many of the apps rely on transferring info or syncing over the same wifi connection - if you don't have that from your hotel or when you're out and about, there are a lot of the 'business' type apps that you'll have reduced functionality on. The Mifi creates a network that you can join everything up to so on- the- hoof syncing works well. I think that even if I get a 3G version one day, I'll keep the Mifi so I have this functionality. I hope developers start using cloud technology soon....
One other point to note if it's useful- even if you think you won't be on the internet much, you end up connected up quite often just transferring files back and forth between applications - far more than I presumed I'd be.
I love this thing - first thing I've been away for a while and not developed a hunchback carting my MacBook Pro around....
Hope that's helpful?