The verse "I'm truly sorry man's dominion Has broken Nature's social union, And justifies that ill opinion, Which makes thee startle At me, thy poor, earth-born companion and fellow mortal!" is just a beauty of construction.
The same poem gave us a phrase that's still in common usage today. When something's gone wrong, people shake their heads and mutter about "the best laid plans of mice and men", but do they remember the source, or the complete line? It's from the penultimate verse of To a Mouse. "The best-laid schemes o mice an' men Gang aft agley", Burns writes (gang aft agley = often go awry). I suppose the modern equivalent might well be "Shit happens" but I'd vote for Burns' choice of vocabulary any time.
Rounding off this remarkable poem, he addresses the mouse thus: "Still thou art blest, compar'd wi me! The present only toucheth thee: But och! I backward cast my e'e, On prospects drear! And forward, tho I canna see, I guess an fear!". Burns wrote of his fears for uncertain, but probably bleak, future prospects more than 200 years ago. Here in 2009, I find he's still bang on the money.