we used one for a cuole of months when we moved house and lost broadband for a while.
used T mobile one - pay as you go. bought it for around £20, I think, this included an offer to top up first month's broadband for £3 (i think)
so, you buy it, plug into USB port, and it installs itself, and opens up a welcome file bit (like an install wizard)
you top up online (it lets you connect to do this bit!) using a credit card, and selsect what package you want (e if offer not on, then our mobile broadband is £2 per day, £10 per week, or 30 days for some other amoutn, can't remember, sorry) there are download limits as well (340 days is 1GB limit with T mobile, BUT once you run out, all that means is that you cannot download between 4pm and midnight. you can still check mail, read MN, catch up with the news, but no video content, or downloading on itunes, etc) outside 4pm to midnight, you can carry on as normal, even downloading.
as far as I know, you should be able to do outlooky stuff - once plugged in, it acts as a normal broadband connection (although not as fast, ime)