Wait what? What is this? You mean the ancient Apocalypse-only one that (
IIRC) wasn't even that good? Or did I miss something, and there's a
40k-legal one with modern rules?
As for the sentry guns, they're mediocre at best. The fact that you can't choose your own target and have to choose between limited arc or limited range is pretty bad, and the reason they're so cheap. And they take up a full
FOC slot (unlike the
HWS, which you can bring effectively unlimited of) for that questionable firepower. Maybe in
RAW 7th, with formations everywhere, that's not such a big deal anymore, but it's a questionable use of a
FOC slot in places that limit formations and try to bring some sanity to the game. On the other hand, they're cheap, and that counts for something.
IMO they're not a bad thing to throw in if you have points and
FOC slots to spare and nothing better to spend them on, but I wouldn't make taking them any kind of priority. I've taken single sentry guns to fill up the last few points and they've usually met my (low) expectations.