I had one eye done last year after a cataract obscured the vision almost over night. The result was a definite improvement but not as good as I hoped and it took months to get used to the side-effects – flickering, some loss of peripheral vision. I would definitely not have a healthy eye treated.
I was advised by the specialist (private) not to have the varifocal type of lens since the results could be a bit hit or miss.
However, based purely on my experience, I wouldn’t leave it too late either – it is probably easier to get a precise measurement of the eye (laser scan) and a better fitting replacement lens if the vision is not too obscured.