If you are brilliant at shifting money around between high interest accounts, OR are absolutely crap with money and just keep spending it, this is not for you.

If however you are just the normal type of person who gets salaries or lumps of cash paid in from jobs and leave some of if it in to pay bills and move some of it to interest paying accounts eventually then this is for you.

One of the main advantages, already mentioned, is you don't EARN interest on your savings so you don't pay tax on anything.
What you do though is AVOID paying extra interest on your mortgage.

Anyone interested really needs to get an individual presentation done for their circumstances.

My mortgage is an endowment type, which was going to be 30% short at final term, doing this offset has avoided me having to pay extra to make up the difference as it will now just cover what's owed.

And I don't work for any financial body or have reason to gain from promoting this idea

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