I've played through the 'insanity' difficulty level in ME2 as an infiltrator. One of my friends, a vanguard class, had a harder time of it since the class relies on suicidal charges toward the enemy anyway.
And some powers most certainly aren't "useless" in harder difficulties. In fact, as an infiltrator, the cloaking power is what bolsters you killing capacity and prevents enemies from targeting you (at the expense of health and shield regeneration). I rarely subscribe to that Game-FAQs mentality of "this is the most effective / best way to play, so you need to play this way." Especially with games like Mass Effect. It bores me.
Edited by MuppetFace - 4/1/12 at 7:07pm