"Sentient" computers would be very scary. Mind, we don't know much about sentience. The singular biggest potential issue with sentient computers is multi-tasking and decision making.
Since we don't know much about sentience, we can't say if our inability to multi-task is due to our sentient nature, or something else.
Also, how much of our reaction time is simply physical (neurons and nerves carry information only so fast) vs our conscious and unconscious minds churning on the inputs to quickly enough give us an output.
It may simply be the fuzzy experience of sentience that makes us go "uh".
If computers are not hindered by either of these, that is able to multi-task at a high level, an reactions limited to propagation delay, then we're in real trouble. Unless, of course, sentient computers don't develop patience. Then they'd probably just go mad "waiting" the eternity of microseconds for use to respond.
A quick reacting, multi-tasking, sentient computer is real trouble. You would have real trouble distracting such a thing, which is our primary weakness in many, many areas.
Simply, imagine these, more intelligent, and coming after you.
Now imagine 100 of them, all coordinated, all with perfect, environmental awareness. They know where you are (assuming they can sense you), they know where all of their brethren are, they know where they are in the ship, or building, or street. If they have wider awareness, they know the surrounding corridors and streets.
Then give them guns.
Give them a gun that they simply have to sweep across you and it fires at the precise point it needs to, already compensating for the fact that it sees you trying to jump out of the way. It's been watching you for that past several hundredths of a second. It not only sees you're turning, it also knows your velocity, and angle of motion, and can predict where you will be when the gun fire 1/10th of a second later, and where that bullet will be in the 1/10th of second it takes to fly toward you, as well as the ballistics of the bullet.
Now, think how much character you want your computer to have.