The Stuff You Care About
(Summarized, because I know you won't read anything beyond this)
It's cheap
It's water and insect-resistant (however; not impervious)
It holds a hell of a lot of weight
It dries insanely quickly
It doesn't twist
Durability
Hemlock is naturally water and weather-resistant; making it perfect for outdoor use
Hemlock is naturally insect and pest-resistant
Hemlock has natural resins that make it burn much slower than hardwoods
Performance and Strength
Much cheaper than hardwoods and most softwoods
Hemlock has an incredible strength-to-weight ratio
Hemlock twists, bows, and cups much less than almost all softwoods
It is a very durable and dense softwood
Hemlock performs incredibly well under load - meaning it resists sagging
Workability
Very easy to plane, sand, and saw
Holds nails easily and secures nails firmly as it dries
Accepts stains and finishes incredibly well
Easier to work with than hardwoods
Appearance
Inherently rustic in appearance - making it perfect for farmhouses, barns, timber-frame, etc.
Unique grain patterns
Knots hold firmly and don't fall out after the drying process
Environment
Hemlock is sustainable
Readily available and regenerates quickly
Cost-effective
"Nerd Stuff"
26 - 28 lbs/ft3 = its fairly light
Janka hardness: 500 (for comparison, pine is 380)
High MOE = it resists bowing
Straight-grain wood = uniform boards/appearance
Natural tannins = resistant to water, insects, and fungus
Well-defined latewood bands = it 'squeezes' nails as it dries
Parallel grain compression: 4,000psi = it can hold immense weight