I Soya I wrote...
I did not read the replys so I am sorry if somebody already said this, but this is from the opening cinematic of ME3. "In 2157, humanity discovered it was not alone in the universe.
Thirty years later, they found a peaceful place among dozens of galactic species. But this idyllic future is overshadowed by a dark past: Reapers, a sentient race of machines responsible for cleansing the galaxy of all organic life every 50,000 years, are about to return. The leaders of the galaxy are paralyzed by indecision, unable to accept the legend of the Reapers as fact. But one soldier has seen the legend come to life. And now the fate of the galaxy depends on him." So yes. They are sentient.
That's a good point, but as I said in a previous post what I should have named the thread was "do they have free will" instead of "are they sentient" because I think A) that's a more interesting question and

it's closer to what I was actually getting at.
Though if the catalyst was really the one behind them all along there would still be A sentience behind them, even if they weren't each individually sentient.