I'm about a quarter of the way though ME3 with my renegade engie right now (which also happens to be my first insanity run.) It is a fun class, but requires a whole other mindset to play effectively and I can see why it's by far the least popular class in SP. It's a shame really since to my knowledge, the engie has the *only* class specific dialogue interrupt in the whole saga.
By contrast, I have about two of every other class...except Sentinel. Which is odd as I had a lot of fun with that one too.