Even Greener Pastures

Irena is absolutely right in definition, but in use it is a borderline word.
When I was a kid there were families that thought any use of that word was obscene, so you had to censor yourself accordingly.  At my house it was ok to use, but it was always a kid's slang.  I suck, you suck, this sucks.
Things are a bit looser today but I think there are better ways to express yourself as the word or phrase can sound grating.  Among close friends it can be a useful way to express sympathy - one friend describes something bad that happened at work or in their private life and another can say "That sucks!", but the use where it fits is very narrow and it is easy to sound childish using it.
Finally, it might sound silly, but one way to evaluate a word or phrase is "Would I hear this in a newscast?".  Would a public official or a teacher use the word in an official statement or in a class?  The answer on suck would be no, there is too much baggage in the word.
Have fun!