Yes I had one for 10 years, concluded this year.
Think of it as a savings account linked to your mortgage. The more in your savings account, the less interest you pay.
Your effective interest rate earned = your mortgage rate. So guaranteed 2-5% whatever you mortgage rate is.
The main benefit for me was the money being accessible the entire time. So I had everything I'd put in available for use at a moments notice.
The downside is that you always feel cash rich and so if you're inclined (or your partner) to spend, might be too much of temptation.
At the start you'd see only a few pounds off your mortgage each month but as that grows and if you still effectively over contribute to your mortgage your saving rate grows exponentially.
At the end of our deal we could continue on variable rate for 15 years, take out new deal at current interest rates or pay off the mortgage. We paid it off.