It's not broken for melee types, but it becomes a must-have for ranged types. Must-have often (but not always) translates into broken. When it really becomes indefensibly broken, however, is when (if?) you start accumulating powers that function as Move actions. It also lets you do some hokey stuff as a melee rogue, coupled with say a fighter. You can hit your opponent every round without fear of retaliation. You shift in, attack, and shift out again (at will). Your opponent is stuck in place, however, because of the fighter and cannot attack you without repercussions (i.e. provoking or readying or other things). Such an enormous amount of new options is definitely a "blast" but lots of options is also often seen as "broken."