Well, it's 75 points more than the basilisk formation, 5 cm slower (artillery don't really move much anyway), +1 armour, +2 FF/CC, slightly shorter range (150cm should still cover it), better guns, an extra small arms attack, an extra macro weapon attack in close combat, fearless, an invulnerable save and walker. Sounds pretty good to me. They shouldn't be shot at much anyway, since they just sit behind your lines in cover and sustain. They'll be moderately hard to shift, since assaulting them is risky and they don't die to some random shots that easily.

Of course, if those Black Legion defiler changes (which I dislike for reasons mentioned in the thread concerning them) are implemented in this list too, they're obviously pretty worthless, losing to khornate daemon engines in pretty much everything except firefight killyness. And the khornates cost 100 points less. Which is another good reason to leave defiler stats as is.