Sentry seems to be intended to stop Recon movement, however, most values for Sentry are less than 15" while Recon models only need to close to within 15". I guess my question is: Is Sentry worth it?
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Situational really, its effective against
Recon
Infiltration
Shapeshifter
Amorphic form
So really it depends on what your opponent has.
You could technically have a model with both sentry and any of those abilities
I think that depends on if anyone in your groups is doing any of the abilities listed by Ravenousminiatures.
My suggestion would be to have a class that has it, especially if you feel you have friends that like to infiltrate. If not at creation, you can always pick it up later through gameplay.
Models with both infiltration and Sentry creates some interesting pre-1st Round activities.