They can be very useful, most people do them too slow IMO.
Use a 3-4 second decend on benchpress, and 4 second decend on the squat.
Also, if you're doing typically 6-8 reps on the benchpress, do 3-4 negatives.
The time under load (TUL) should be approx the same.
Don't do them every week either IMO. Either every other week, or every third.
And perhaps one or two sets.
When used for powerlifting training: Use it during off-season training IMO. Negatives will recruit Type IIa fibers, and not IIx.
JMO.
PS Get as much help as possible from the spotter/s on the way up. You want to lift as little as possible in the eccentric phase on these sets.