I liked sentry at low gear level/while leveling. They are the best sustained damage spell you can use in the game. Granted depending on the difficulty you play at, and especially if you play at the "kill everything in one shot" difficulty, then they're not useful, but for anything harder, sentries are worth it. They're worth it if they get to do damage to a mob for 6seconds or something, I forgot the math. they're especially good when killing story bosses, like for bounties or well when doing the story.
Should definitely not build around them though. It's something you take as bonus damage to your main source of damage, similar to the wolf or marked for death. The only time you will use sentries as your main source of damage is once you have marauder so they actually do good damage on their own.
I wish they'd change sentries to be charge based for 2.1. This would make them so much better even without the set. Being able to drop 2 sentries at once every 12seconds instead of one every 6seconds would be a massive quality of life improvement and would definitely dwarf most other skills in terms of doing burst damage for elites.
Basic build would be one spender/builder of your choice, vault tumble+tactical advantage, sentry(either piercing or rocket, rocket being better for bosses and elites with no mobs around, piercing being better in spammy situations), companion wolf(before it's available, spider with cull the weak is pretty solid, or boar if playing hardcore) and a skill of your choice(preparation, marked for death, vengeance, rain of vengeance, whatever really). Don't recommend cluster for leveling unless you have a cindercoat, it's rough to run without the cost reduction, chakrams are pretty good though(razor disk or poison), frost elem arrow is also good.
