I have been considering an offset mortgage for a while now and done a bit of research into it, however my siutation is a bit different.
I am currently living abroad and returning to the Uk in early 2007, i rent my house out ( positive cashflow a tenner a month!!!) and i will return with a substantial amount of savings in relation to my mortgage. For me the savings will be around 60 -70% so its a no brainer.
From what i see the advantages are as follows:
1. Flexibility
You agree an overdraft equal to the valuation of your house with the option to draw on this overdraft at any point in time but interest does accrue on this extra amount. This assumes that you have equity in your home. You can also reduce or increase payments very easily.
2. Save interest
The interest is calculated on a daily basis against the balance of your account, not just the lump sum.
eg
100,000 loan
20,000 saving
paid 2000 first day of the month
balance = 78,000
Pay interest on 78,000 for that day not 80,000.
So if you pay your wages in to the account this too will be offset as the interest is calculated daily. (as aussie bloke said) saving you more interest. However you do sacrifice the interest that could be earned in the bank.
3. Laziness
If you are lazy like me and cant be bothered to change mortgages every 2 years to save interest then it may suit. If you dont transfer over spare cash each month out of your current account and let it grow you will only be getting interest at less than 1%. In this situation you do nothing and the money works for you by saving cash.
In summary i'd say its for lazy people like me, with a fair amount of savings who want the flexibility of being able to underpay and overpay thier mortgage.