it is isn't it, I changed all the batteries in my rear lights last weeks and yikes! It's defo worth changing rechargeables every week as they'll still be bright but nothing like as bright as they can be
the now quite old cateye 10 led one I have on the seatpost is quite the ball of fury and light too, it's all so bloody god compared to the old days
I still have front and rear basta lights from at least twenty years ago in my loft, ate 2 C/D batteries every week in return for a few hours of glimmer