I disagree with most people, I think they can be fantastically effective.
They're fairly tough, good in combat (despite not being armed for it), and have powerful guns (you would ALWAYS give them More Dakka!).
If I were to take them, they would definitely be outfitted with both Dakka and Blasta, meaning that have a 66% chance of being AP3, and a 50% chance of being AP2.
That said, they are effective when used right, mobile, and tough; but the main problem with then, as has been mentioned, is their FOC slot.
They only really have a place in a very specific army, to make up for a huge lack of anti-MEQ, and even then the enemy doesn't always take heavily armoured dudes.