Had one for a number of years - worked very well....
You can look at it in a number of ways - if you get bonuses/commission payments from time to time, as soon as they hit your account they reduce your overall mortgage amount (and thus any interest payments due) and reduce your time to repay. Overpayment becomes the norm rather than something you make an effort to do - whatever is left in your current account just goes against your overall debt.
There is nothing to stop you paying lump sums in, benefitting from the reduction in interest payments and then drawing it out again when/if you need it.
I calculated that the interest I would earn in a (admittedly basic) savings account would not be more than the interest payments I would save by putting the savings I had against my mortgage.
There are better rates out there, but for me and my spending/earning profile it works out very well.