The main thing is (as I see it) that they change the rate at which interest accrues at the same date as the rate change (and to be fair, it looks that they did do that as best as I could check online); I'm not too bothered about the actual monthly repayment lags a little in reflecting the new rate (every extra euro you pay off now will save you interest in the long term).
Case in point:
I owe 295k with 21 years left to run, repayments at 4.75% are 1852 (to nearest euro). Repayments at 4.25% should be 1772 (again, to nearest euro). By keeping my payments at 1852 (in effect, overpaying by €80), knocks around 12k off the total paid to the bank and 17 months off the term.
If I were to throw in the TRS against the mortage too (soon to be €75, so overpaying by €155), that knocks off around 21k in interest and 31 months off the term.
My plan is, for as long as I can, to keep the payments up to a notional interest rate of 4.75 (what I've become used to paying) and chuck the extra at the mortgage. If the ECB were to reduce rates tomorrow by another 50 basis points, then my repayments would fall to 1693 and overpaying by €234 knocks nearly 4 years off the mortgage.
There are a good few online repayment calculators around, try googling "" and plugging in your numbers, see what you find.