I would tend to agree that the sentinel armor isn't very useful.  You get less DR than assault armor.  The trade being that you get the turret ability.  The turrets don't have much health, so they get wasted pretty quickly against stronger enemies (not much value) and they spend a long time on target acquisition so they likely won't pick up a less powerful enemy before you have time to eliminate it yourself (again, not much value).

The only time that I have ever found them to be moderately useful is on the data hunt missions where you activate a cogitator and hold a key (default "F") for a while. and missions where you defend a static point  The turret can help hold off a few enemies, but not for long unless you keep enemy fire off the turret.  The turret gets destroyed quickly if you let an enemy focus on it and it spends most of its time spinning around, and not shooting, trying to decide on a target.

I used it at lower levels where I would draw a mob and let the turret(s) mow them down. When I got relic gear I can handle the mobs no problem. I use rockets now.

I used the armor once got upset at how low health its had, how it could not aquire some targets, and how much time it took to use again and switched back to the rockets but I do like the Jump pack too. I think the turret needs to be buffed