A grinder is the machine that has more effect on the espresso than the espresso
machine does. This is not a popular opinion where espresso machines are
expensive.
What the grinder does
It cuts beans into pieces that are all roughly the same size. A lot of the
pieces will be finer than you want and a lot will be coarser than you want, but
the average will be the size you set.
The better the grinder, the smaller the spread. A machine that costs $50 cuts
everything into a range from powder to chunks. A machine that costs $300 cuts
everything into a tighter range. A machine that costs $1000 cuts into a very
tight range.
The effect on the cup
A tight range of particles extracts more evenly. Even extraction tastes clean.
Uneven extraction tastes muddy or sour depending on which way the unevenness
goes.
Which grinder to start with
If you drink espresso three times a week, a grinder that costs $150 is the
minimum. It will be loud and slow but the grind will be consistent enough to
learn from.
If you drink espresso once a day, a grinder that costs $300 to $400 is where
you start to enjoy the coffee and stop fighting the machine.
If you drink espresso more than once a day, you probably already have a grinder
and you know this better than I do.
The grinder I use
A grinder that costs $500, and it is the second-best machine in my kitchen. The
espresso machine is the first because it is a ritual. The grinder is the one
that makes the espresso taste good.
The one I used to have
A grinder that cost $100. It made bitter espresso because the fines (the tiny
particles) were too much, and the fines came from the blade cutting unevenly.
One day the espresso suddenly tasted better. I did not change anything. I
cleaned the chute and the grinder died. I bought the $500 grinder and
everything changed.
The question that matters
“Will I change the grind throughout the day to match the beans?” If yes, you
need a grinder good enough that changing the grind is fast. If no, a cheaper
grinder is fine.
The grinder that takes three minutes to dial in between shots is a grinder you
will not use, and the espresso you do not pull is the worst espresso you can
make.