Can either be used as an expression for good or for bad, although usually to denominate something particularly good.
Used in place of good, great, awesome, bad etc.
The phrase was originated on the forum, "The Pit", by (now) esteemed user, Stratoblaster73, and has now spread to be a commonly used term not only on the tubes, but in real life as well.
1. Holy shit The Dark was tits on a whale, man! (good)
2. Eughh, dude, your girlfriend fucks like tits on a whale. (bad)
1) An exclamation of surprise or shock, similiar to dayum.
2) A dismissal of something unhelpful or useless
1) A) I just won a new car!
B) Tits on a whale, that's sweet!
2) A) Hey guys, I can play "flight of the bumblebee" at 320bpm!
B) Yeah, except you've wasted your life learning a skill which is as useful as tits on a whale.