I 'm not going to use all time :) , but wanted to know how strong it is and when/where/ with whom it can be used.

Indeed! There's also a sexually vulgar element to it (which some speakers conveniently deny). Some of the more colourful versions of "it sucks" do highlight the vulgarity. Are you sure you want to use it? :)

Is it a kind of insult?

It's American slang, meaning 'he's useless' [at something], 'it's bad', 'it's useless'.