We run a mainly pastured raised operation. We use rotational grazing, haylage, alfalfa, high quality mineral, and bolusing (2-3 times a year) in order to maintain condition and milk quality/quantify. Grain is only given as a treat.
As those previously have stated, it really depends on the quality/amount of pasture, genetics, and your expectations. Our commercial herd is comprised of San Clementes which have not be improved like the traditional dairy breeds. Also, we breed/cull heavily for those who do well on pasture only.
For our farm, it has worked well, but we knew this was our goal and planned accordingly. I can easily see where it would not suit others. You need to weigh the pros and cons for yourself.
Good luck!