Offset mortgages are very useful, provided the interest is calculated daily. Your normal expense accounts eg. Cheque with say, $4000 then offset and reduce your interest, even if you don't have big savings. Even small amounts can end up saving months off a term.
The downside is that these are floating interest rates, so can change at any time. You can then fix if the interest rate starts to get too high, but you lose the offsetting benefit.
I encourage my clients to have atleast a small amount on offsetting because I've seen how much it can save people in the long run.