Hi...the difference between blackberry internet service (the £5 a month bit) and the unlimited internet add on is that the unlimited internet is paying for the actual data that you use...there the blackberry internet service is paying for use of the blackberry specific services...and unfortunately you do have to have both in place to use the blackberry properly!
Without the BIS you will not be able to access email, messenger or the internet browser and without an unlimited web add on you can access them but the amout of data used would cost you a fortune!
It's well worth paying for though, blackberry email delivers all your email the instant you receive them, blackberry messenger offer always on access to instant messaging with all your friend who have blackberry's (you can also instantly transfer files and photos etc) and you also get access to the app world and use of apps like facebook and twitter which are really fast and again all notificationd are pushed straight to the handset so you never miss a beat!
Hope this helps!!!
