I recently got a monk to join my campaign at level 12, so I can't say how heroic tier works, but for the 3 or 4 combats he's been in so far I'll say that the monk, when built with some forethought is pretty dang beefy. Maybe not the best damage output, but he's got the highest AC in the group. Most of this stems from a series of feats he grabbed to make hitting him nearly impossible in certain situations. He's got the following, if I'm remembering it all correctly:
Deadly Draw: When you slide/pull an enemy adjacent, gain CA against that foe.
Unarmored Agility: +2 to AC
Defensive Mobility: +2 to AC vs OAs
Defensive Advantage: +2 to AC against enemies he has CA against.
Opportunistic Withdrawal: Doesn't provoke OAs for moving from enemies he has CA against.
Eyes in the Back of Your Head: Doesn't grant CA when flanked.
He also has Boots of the Fencing Master, which is another +1 to AC and Reflex nearly every turn.
The first turn he moves up to one baddie and hits it, slides it with FoB next to him, which gives him CA due to deadly draw. At this point he's at +2 AC (+3 if he shifted from the power), and can get out of there without worrying about OAs since he has CA.
Not only that, but enemies can mob up on him, and he not only doesn't grant CA for flanking, but he most likely has +3 AC from having combat advnatage and having shifted, and can just saunter away without taking any opportunity attacks (maybe 1 or 2, but he still has +2 AC vs those).
Needless to say, he's about as hard to hit as a paladin, even if his damage output is mediocre (not to mention all the shifting he does to enemies).