(for those over 30 years old) Always a transitive verb. Colloquially used as shorthand for suck dick with noun dick understood. Synonymous with blow and bite. Used as an insult; indicates homosexuality.
Cracka 2: So, what's your point?
(over 30) Son: Gee dad, I don't want to do my homework. Math sucks.
Father: You're grounded for a week, you perverted little delinquent. And wash your mouth out with soap.
Not to confuse 'suck' with 'blow' as in: The Steelers blew like a third world prostitute.