Forcefields are strong, but sentries lose their place in the army after the 10 minute mark. Before then, treat sentries as a threat; if you overstep i will cut your army in half and kill it. Sentries are really weak and do almost no damage on their own, not to mention force fields are ineffective against massive units. Once terran gets ghosts, sentries only take up space. Once burrow/burrow movement is researched by zerg they can just stall or move past the forcefields.
forcefields are fundamentally overpowered, and i have always thought that, but starcraft is a game about balance based on overpowered. Look at the defiler, eating a ling to cast a spell that makes all ranged attacks worthless. Dark Swarm is overpowered. Reaver Drops, BW siege tanks, Lurkers, BW psi storm, Mind Control, BW mutas, medics. All that stuff was super strong, but because every race had lots of stuff that was really strong, it made for lots of good strategies and in the end lead to a very balanced game. Balance from Imbalance.