They have a cost of 2.3 marines. They are very good as Saim-Hann, not so much as anything else - it's the Saim-Hann stratagem that makes them devastating.
Per their base stats and cost, they are a very solid highly efficient unit, worthy of a place in a highly competitive list. Thing is though that they have to get really close to the enemy, and being close to the enemy is highly detrimental for one's life expectancy. They can be killed fairly easily with any multi-D weapon, which are plentiful in any properly optimised list. I've been proxying a unit of 6 in my games lately (my actual spears are still awaiting their turn to get painted), and in most of my games they assault and kill something on turn 1, then die in my opponent's next shooting phase.
There's one thing that seems to make them pretty OP though, and that thing is a warlock with Protect.