I can see Sentry being useful for kite-bosses like Siegebreaker, or possibly even Azmo (thinking in terms of Act III right now).
It's not very helpful in Belial as you tend to want to stand in its beneficial bubble, which is utter trash no matter what you pick.
Why is it utter trash?
1% health is nothing. If I have 40,000 health, that means I heal 400 every second. If I have that much health, I'm most likely doing Inferno or Hell where I take upwards of 80,000 or 30,000 per hit, respectively. Again, I'm thinking in terms of Act III. Azmo-Ball hurts, even in Hell.
You're also confining yourself to a small circle letting yourself get easily hit.
As for 15% damage reduction. Let's assume you're taking 30,000 damage and you have 40,000 health. 15% of 30,000 = 30,000 * 0.15 = 4,500 damage reduced.
40,000 - (30,000 - 4,500) = 15,500. It lets you live for 2 shots which you would have lived through anyways.
The damage on the Sentry is utter trash. 20% is nothing, I get more out of Ferrets. Even with Rockets, it's still trash. Unless you're using Marked for Death (Mortal Enemy), I wouldn't even consider Sentry. Even then, I'm still reluctant to put it on my bar as it's a pain to lose two slots in Inferno.