depends, by sentries I assume you mean sentry guns? (tarantulas)
Tarantula platforms are iffy, the rules on targeting means you haev to pick one of 2 modes and neither exploits the full range and they are relatively easy to take out (low armour and any glancing or penetrating hit kills

I am considering getting some for my drop troops, simply cos they lack heavy weapons, but the targetting stuff does nerf them

As for turrets, I would consider if I didnt use mechanised cadian and a drop troop army, neith of which realyl fits with turret emplacements

 

And lets not forget that sentries are stationary weapons so anything canpretty much shoot at them. They are hard hitting but VERY extremely weak when it comes to taking shots

 

i know they die easy but they are really cheap (15 pts)

turret emplacements look at it this way why have a leman russ to move around and getting shot to pieces when you can have one standing far away to only 130 pts