I want to point out that there's nothing /wrong/ with the starter Napa etc. sells - they work just fine and will crank plenty fast.
However, 'cheaper' starters like the autozone ones might not last very long - build quality isn't great.
The design of the starter is fine, though, and it /is/ a gear-reduction unit.
I also believe in cleaning your own starter and making sure it is shot before replacing it - often it just needs some cleaning, new grease in the gears and it'll work like new.
The main wear-out part is the brushes - if 1 of the 4 gets too small, the starter runs on half power, which acts like a dead battery(slow cranking speed)